Louis van Gaal has been accused of playing "dull and functional" football at Manchester United and of having a "baffling" transfer policy by former England striker Alan Shearer.
Shearer launched his outspoken attack on Van Gaal as the Dutchman prepares for Saturday's crucial meeting with old rivals Liverpool at Old Trafford.
United have failed to convince in the Premier League this season. They won their opening two games - both by a single goal - but were then held to a draw at home to Newcastle and beaten at Swansea.
While they have made progress to the group stage of the Champions League, courtesy of a play-off win over Club Brugge, Van Gaal has also been criticised for his style of football, with United scoring just three goals in the league this season. Only three sides - Newcastle, Watford and Liverpool - have fewer.
And Shearer believes United's paucity of play, combined with their unconvincing form last season, means Van Gaal cannot afford a defeat in Saturday's meeting with Liverpool.
Writing in The Sun, he said: "I would argue Louis van Gaal cannot even afford to draw it.
"He has been asking for more time for his team to blend, for young players to develop etc. Time is up I'm afraid - the time is now.
"You cannot oversee the changes he has made, backed by hundreds of millions, and then claim you are a victim because it needs time to blend. It is your job to mould a side and I would suggest a year is plenty for a club of Manchester United's standing.
"You can get away with the dull, functional football Van Gaal is dishing up if you are getting results. But if you go back to the end of last season Van Gaal's league run over the last 10 games now stands at three wins, three draws and three defeats.
United's activity in the transfer market has also been subject to question: after missing out on Pedro, Neymar and Thomas Muller, United decided to spend up to £57million on Anthony Martial, a 19-year-old with just a handful of goals to his name at Monaco.
Shearer believes that policy could be disastrous for United this season, and suggested that had David Moyes - Van Gaal's predecessor - made similar decisions, he would have been pilloried.
He added: "His recent attempts to change that in this summer's transfer window have been baffling. He has started Adnan Januzaj in four games this season - and then shipped him out. He tells Javier Hernandez he has another chance and then sells him.
"Then in comes a relatively untried teenager in Anthony Martial upon whom there is now a huge expectation after a deal which will eventually be worth £57.6million.
"Van Gaal's CV has afforded him the time that David Moyes was not given. But a poor result on Saturday and questions will start to be asked."