The Spanish playmakers were team-mates at Valencia and have been two of the most influential players in the Premier League this season and Mata is still confident that Chelsea can cut down City's lead at the top.
Mata told reporters: "He's my friend. He's doing very well, playing for Man City and I will change my shirt with him.
"This season has been very, very good for him because he's showing his level but I'm just thinking of winning.
"Man City are having a great Premier League.
"There are so many weeks until the Premier League finishes. We will keep going every week to win and to be closest to the leader."