The Spaniard is nearing a move to the reigning Premier League champions after appearing at the club's training ground earlier today for a medical ahead of a £37m switch from Chelsea.
Solskjaer, who enjoyed an 11-year spell at United, will take his South Wales outfit to Old Trafford on Tuesday.
Although the Red Devils legend is enthused about his former club's proposed acquisition of Mata, the Norwegian joked that he would prefer to see the 25-year-old make his debut after the visit of the Bluebirds next week.
Speaking to reporters following his side's 1-0 victory over Bolton Wanderers in the FA Cup, Solskjaer said: "He is one of the top players in European or world football and he has done fantastically well at Chelsea.
"Manchester United have signed a top, top player. But I hope he will have to wait until next week to play."
Frazier Campbell's 50th-minute strike proved enough for City to progress to the fifth round of the competition.