Louis van Gaal "doesn't understand English football" and is "not a head coach for Manchester United", says their former winger Andrei Kanchelskis.
But Kanchelskis believes the Dutchman should remain in charge until the end of the season, despite a six-match winless run in all competitions.
Van Gaal walked out of Wednesday's news conference in protest at media reports that his job is under threat.
"I am disappointed with the way United are playing," Kanchelskis said.
"Under Alex Ferguson, we played in a more attacking style of football and played in a 4-4-2. Van Gaal sees things differently. This is not a head coach for Manchester United.
"I just don't think that Van Gaal understands English football. A team needs to have leaders, like Bryan Robson, Roy Keane, Peter Schmeichel, who can give the right things at the necessary moment and can show their leadership qualities.
"The coach does not want this and this is the reason for such disastrous results."