United are aiming to sign Mata this week.
"I like Juan Mata because he is a match-winner on his day," McIlroy told the Manchester Evening News.
"He is a little bit different to what United have at the moment in midfield and is just what we need.
"The one thing people are asking is why is a manager of Jose Mourinho's calibre and success getting rid of him?
"Why does he not fancy him in his team when he has been so popular and successful in Chelsea's midfield in the past?
"Some reckon it is because you have to build a team around him because he cannot do the tracking back and work that Jose wants.
"But if you have to build a team around him why not? If others have to do a bit more work to accommodate him, what is wrong with that? Sometimes you have to go for a player some might regard as a 'luxury'.
"It all depends on to what extent you 'carry' a player. If he is a bit special and wins you matches then sometimes you can afford to have that kind of a player in your team."