The Armenia international has been enjoying an incredible playtime this season, having scored twice and provided six assists so far.
Manchester United are being linked with a move for Ozil, who looks increasingly likely to leave Arsenal in the near future.
The Germany international's contract expires next summer and, despite the club's best attempts, the 29-year-old appears to have no intention of extending his stay.
Mourinho managed Ozil while at Real Madrid, signing the playmaker from Werder Bremen and transforming him into a household name.
But, while the 54-year-old is an admirer of the Arsenal No10, Castles says Mourinho does not deem him an upgrade on those already at his disposal.
"Mourinho does like him as a player. He's got that combination of creativity and tactical awareness that he likes," Castles said on the Transfer Window podcast.
"But the information I'm getting is that Ozil's not the player he plans to add to the team next summer.
"He's already worked out the areas he wants to improve in.