Rooney, who has lost his England spot to Defoe following a disappointing season at United under Jose Mourinho, has been strongly linked with a summer exit from United.
"I love Wazza. He's an unbelievable player. Someone who's done so much," Defoe said.
"I read things about him and sometimes I'm quite taken aback and think 'has everyone forgotten what he's done in his career?' He came into the game young and played a lot of football. It's not easy on the body, and mentally with all that pressure he's under - probably more than most of the lads.
"I think he knows he's got to play regular football. All I'd say is I wish him the best. He's a top player and the lads love him here. Hopefully he'll get the chance to go somewhere and express himself, and just enjoy playing."