Donaldson glanced home the Bees' second goal with five minutes left to add to Toumani Diagouraga's opener just after the half hour.
Rosler said: "That is typical of Clayton. He doesn't score easy goals. He is best when he does it by instinct and that was a great goal. He deserves it because although he gets a bit of criticism, his workrate has never dropped and his pace is always a threat to defences."
Rosler, whose side ended a three-game losing run, also singled out midfielder Jonathan Douglas for praise.
Rosler added: "For the job he does and the work he puts it I'd say he is the best, most consistent player in the league.
Brentford lost on-loan Everton midfielder Adam Forshaw to an horrific jaw injury just before the half hour, but replacement Diagouraga broke the deadlock minutes later.