Jose Mourinho has behaved "like a big baby" and Manchester United will only improve under a different manager, says Premier League winner Chris Sutton.
Mourinho's United lost 3-1 to derby rivals Manchester City on Sunday, leaving them eighth in the table.
Sutton said those comments were "disrespectful" and "utter rubbish".
City beat Southampton 6-1 in the league and Shakhtar Donetsk 6-0 in the Champions League before playing United, who defeated Bournemouth 2-1 and beat Juve by the same scoreline.
Speaking on BBC Radio 5 live's Monday Night Club, former Blackburn and Celtic striker Sutton added: "United were outclassed. It was like watching Manchester versus Manchester reserves.
"There is an enormous gap between them and City, and things aren't going to change unless they change the manager.
"What was most galling for me were Mourinho's comments after the game - kicking off, behaving like a big baby. When is it going to stop?"
United finished second behind City last season, and Mourinho said repeatedly in the summer that the club needed new signings if they wanted to compete for the title.
Brazil midfielder Fred was their only major recruit, in a reported £47m move from Shakhtar Donetsk. The Red Devils also signed 19-year-old defender Diogo Dalot for £17.4m from Porto, and third-choice goalkeeper Lee Grant from Stoke.
"Mourinho has been banging on about money but is United's form this season acceptable on the back of last year?" said Sutton, who won the Premier League with Blackburn in 1995.
"He's been talking about how much City have spent but they have to catch Watford and Bournemouth first.
"And when you look at the signings he has made, how many have actually been a success?
"Has he spent the money well? I don't think so. Why should the United board trust him to spend well in January?"