This article directly refutes Gustav Aulen (by name) by citing Luther:

https://guardthedeposit.com/2013/03/martin-luther-atonement-penal-substitution/

The article quotes "a passage from Luther’s Second Sermon on Luke 24:36-47, where the theology of penal substitution comes through clearly:"

But now, if God’s wrath is to be taken away from me and I am to obtain grace and forgiveness, some one must merit this; for God cannot be a friend of sin nor gracious to it, nor can he remit the punishment and wrath, unless payment and satisfaction be made. Now, no one, not even an angel of heaven, could make restitution for the infinite and irreparable injury and appease the eternal wrath of God which we had merited by our sins; except that eternal person, the Son of God himself, and he could do it only by taking our place, assuming our sins, and answering for them as though he himself were guilty of them. This our dear Lord and only Saviour and Mediator before God, Jesus Christ, did for us by his blood and death, in which he became a sacrifice for us; and with his purity, innocence, and righteousness, which was divine and eternal, he outweighed all sin and wrath he was compelled to bear on our account; yea, he entirely engulfed and swallowed it up, and his merit is so great that God is now satisfied and says, “If he wills thereby to save, then there will be a salvation. (Sermons of Martin Luther, vol. 2, p. 344)

If I may crudely summarize, Gustaf Aulén's argument in Chapter 6 of Christus Victor is:

1. Martin Luther clearly valued the Christus Victor theme
2. The Christus Victor and substitutionary models of atonement are fundamentally contradictory
3. Therefore, any references to "satisfaction" or Christ bearing God's wrath on behalf of humanity must be understood in light of the Christus Victor paradigm.

No one disagrees with #1 – the idea that Christ is victorious over sin and death is not controversial and thus it's not surprising that not only the reformers but also modern-day Protestants recognize this theme as an important aspect of Christ's work.

Aulén gets in trouble with many critics, however, when he argues for #2 and #3. They argue that the Christus Victor theme is not inherently at odds with substitutionary atonement, and therefore that Luther's language regarding satisfaction can be taken at face value.

Aulén on "satisfaction" in Luther

Luther's appreciation for the scriptural theme of Christ's victory over sin and death is obvious, so Aulén easily makes that portion of his case. He must, however, deal with the places where Luther uses words like merit and satisfaction:

Luther uses both these terms, Merit and Satisfaction, in direct relation to Christ's conflict and His victory over the 'tyrants.' (118)

Christ's merit, he says, is for Luther simply a way of talking about God's grace, and satisfaction similarly is meant to emphasize the "strength of the Divine Love, which could go in under the punishment that impended upon men." With this understanding, Aulén feels that he can sufficiently explain Luther's use of language associated with substitutionary atonement, as in the following quote from Luther:

We firmly believe that Christ, the Son of God, stood for us and took all our sins upon His neck, and is the eternal satisfaction for our sins, and made atonement for us to God the Father; he that believes this has a place also in this sacrament (Holy Communion), and neither the devil, hell, nor sin can harm him. Wherefore? Because God is his defence and his helper, and if I thus have believed, therefore I know surely that God fights for me, in spite of the devil, death, hell, and sin, which would harm me; this is the great inestimable treasure which is given to us in Christ. (119)

Thus, he says, Luther's view of the sacrifice of Christ is best expressed as follows:

It is God's act of victory, when Christ goes in under the Divine wrath, and bears the burden of the punishment which on account of that wrath impends upon men. Thus the Love of God breaks through the Wrath; in the vicarious act of redemption the Wrath is overcome by the Love. (115)

Aulén's critics

Aulén's analysis is, not surprisingly, widely challenged. Critics argue that Aulén fails to recognize the compatibility of a Christus Victor theme with penal substitution, and point to numerous instances of Luther using the language of the latter. On the first point, we have Timothy George in The Glory of the Atonement, 275:

Many scholars have criticized Aulén for drawing too sharp a dichotomy between the idea of penal substitution and the motif of victory over the evil powers. We need not accuse Luther of being inconsistent for having seen the truth—that is to say, the biblical basis—for both views of the atonement. Indeed, this may well be Luther's major contribution to atonement theology: just as he brought together the ideas of satisfaction and punishment in the doctrine of penal substitution, so too he saw that the cross of Christ was at once the scene of Satan's definitive defeat and the objective basis of justification by faith alone.

And in support of Luther's acceptance of penal substitution, proponents point to a variety of passages, especially several in his Commentary on Galatians (Luther's Works, ed. Pelikan, v. 26):

[Jesus] has and bears all the sins of all men in his body—not in the sense that he has committed them but in the sense that he took these sins, committed by us, upon his own body, in order to make satisfaction for them with his own blood. (LW, 26:277, quoted in Defending Substitution)

[The Father] sent his Son into the world, heaped all the sins of all men upon him, and said to him: "Be Peter, the denier; Paul, the persecutor, blasphemer and assaulter; David, the adulterer; the sinner who ate the apple in Paradise; the thief on the cross. In short, be the person of all men, the one who has committed the sins of all men. And see to it that you pay and make satisfaction for them." Now the Law comes and says: "I find him a sinner, who takes upon himself the sins of all men. I do not see any other sins than those in him. Therefore let him die on the Cross." (LW, 26:280, quoted in Defending Substitution)

If the sins of the entire world are on that one man, Jesus Christ, then they are not on the world. But if they are not on him, then they are still on the world. Again, if Christ himself is made guilty of all the sins that we have all committed, then we are absolved from all sins [...]. But if he is innocent and does not carry our sins, then we carry them and shall die and be damned in them. (LW, 26:280, quoted in Defending Substitution)

For you do not yet have Christ even though you know that He is God and man. You truly have Him only when you believe that this altogether pure and innocent Person has been granted to you by the Father as your High Priest and Redeemer, yes, as your slave. Putting off His innocence and holiness and putting on your sinful person, He bore your sin, death, and curse; He became a sacrifice and a curse for you, in order thus to set you free from the curse of the Law. (LW, 26:288; quoted in "Penal Substitution in Church History")

But because an eternal, unchangeable sentence of condemnation has passed upon sin – for God cannot and will not regard sin with favor, but his wrath abides upon it eternally and irrevocably – redemption was not possible without a ransom of such precious worth as to atone for sin, to assume the guilt, pay the price of wrath and thus abolish sin. This no creature was able to do. There was no remedy except for God's only Son to step into our distress and himself become man, to take upon himself the load of awful and eternal wrath and make his own body and blood a sacrifice for the sin. And so he did, out of his immeasurably great mercy and love towards us, giving himself up and bearing the sentence of unending wrath and death. ("24th Sunday after Trinity"; also quoted in Gregory Allison's Historical Theology)

Summary

The interaction with Gustaf Aulén's Christus Victor continues to this day, lending him credibility despite the criticism. Some have followed in his tracks, like Phillip S. Watson in Let God be God. Others have concluded that the traditional understanding should be retained, as expressed by Wolfhart Pannenberg, who said that Luther saw "with full clarity that Jesus' death in its geneuine sense is to be understood as vicarious penal suffering." (Jesus: God and Man, 279)

Thus the answer to this question is perhaps not as simple as we might like. Neither demonstrating that Luther completely rejected penal substitution, nor showing that he strongly preferred it over all other models, are easily accomplished. Perhaps he did not express penal substitution as systematically as John Calvin or Charles Hodge (though that could be said about many of his teachings). Perhaps his writings bear out a preference for Christus Victor over other models. But, to me at least, Aulén's fails to convincingly prove the incompatibility of the two models in Luther's mind and thus his complete dismissmal of penal substitution in Luther seems excessive.