Modric joined the defending La Liga champions from Tottenham last summer, but has endured a difficult first season in Spain - and was even named one of the worst signings of the campaign by a Spanish newspaper earlier this term.
But Macari believes the Croatian international could be the perfect summer signing to strengthen the Manchester United midfield next season.
When asked by talkSPORT what he thought about a prospective Luka Modric signing, Macari said: "Well, that's a possibility, especially if Jose Mourinho leaves Real Madrid. Modric won't want to stay there then, it's a good shout."
United's need for a new central midfielder has often been mooted, with Ryan Giggs and Paul Scholes approaching the end of their Old Trafford careers, and Anderson struggling to stay fit.
Giggs was at fault for Manchester City's first goal in Monday night's derby, and Macari believes he was swamped in the Old Trafford midfield against Roberto Mancini's side.
"Ryan Giggs has been fantastic for the last couple of months but against Man City he was over-run. It's the first time in a couple of months that he's played against top quality players in that department," added Macari.
"Man City have got a very good midfield unit. Giggs has been great but how long can he go on for? He's getting near to the end. For that reason alone the manager will look at that department this summer to bring someone in."
Macari was an Old Trafford fan favourite during his 11 years at United, during which he made over 300 league appearances between 1973 and 1984.