Players fit systems. Always have, always will. There is no such thing as a universal player.

Donald does NOT fit every system. He is a 3 technique DT. Period. He is not long enough to be an effective 4-3 DE, for instance. He is not a 3-4 NT.
Look at Julius Peppers, a future HOF DE. He made the grade as a 4-3 DE. He cannot play DE/DT in a 3-4 without making him average; he needed to be moved to OLB. Can you imagine putting Tom Brady in an offensive system that required him to be mobile and even be part of the run game to an extent? Like the Seahawks, or the Chiefs? You don't get any greater than Tom Brady.

If it is true that Phillips will use Donald the way he's always been used, then he will mainly be effective as a situational player when they put 4 at the LOS and he plays over a Guard.

Just watch. His years of 9 and 10 sacks are over with the Rams unless they try him at OLB (which I kind of think he might be able to play).

He needs to be traded to a 4-3 team RIGHT THE HECK NOW, because he is NOT going to have a very good year, which will diminish his market demand. You cannot miss all OTAs, all Training Camp, all practice, switch positions and learn a new Defense and expect to be very effective in a matter of days. They should approach Dallas, for instance: Donald for Zack Martin and a 2nd round pick. Cleveland for a 1st and a 2nd rounder. Something like that.