If you're asking this question honestly... causing immediate retirement is not really the point, but Ben Bishop and Carey Price immediately come to mind as elite goalies who literally retired / stopped playing because of knee problems. Just as often, players come back and aren't able to stop the puck which is especially relevant for a guy like Forsberg who's just recently put it together as an NHLer.
Knee injuries for goalies are serious, basically the same level as hip injuries for forwards -- more relevantly the chance of the knee injuries affecting Forsberg's play is significant.
Even if he comes back, if the rehab doesn't lead to 100% recovery (more than likely), he can't move around from his knees which is very important for goaltenders who are often in the butterfly / rely on knee torque to quickly get in or out of that position as well as move around on the ground. If so, well, probably he won't be able to be counted on to be an NHL starter.
Getting MCL injuries in both your knees is extreme, coming back from one MCL injury is bad enough but I can't think of an example where it's happened in both knees at the same time. Can you?