Upvote:3
When an airline sells you (or a re-seller) tickets for a multi-stop flight, they imply that it is reasonably possible to change planes.
Brussels Airport is not very large (in comparison to, say, Atlanta), so walking distances are reasonably short. Nevertheless, for any connecting flight, it's usually a good idea to go straight through all security controls until you're sure to be within a few walking minutes of the connecting gate before you start spending time on exploring the airport.
In case your flight from Manchester gets delayed enough so that you cannot make the connection, they will either have somebody waiting at the gate you arrive at from Lufthansa you can talk to, or you go the Lufthansa counter and explain your situation, and they'll book you on a later flight.
In case the flight gets cancelled in the first place, go to the Lufthansa counter, explain your situation, and they'll find an alternative for you.
As a general piece of advice: relax when traveling. Sometimes, things will go wrong. Roll with it. Don't ask "Why the hell did you idiots close the airplane door right as I was running down the hallway waving at you". Ask "Now that I missed my flight, can you please help me reach my destination" - and enjoy that you have just been gifted some me-time.