Paul Pogba is currently at his peak and will not live up to expectation at Manchester United because he is too inconsistent, according to Sam Allardyce.
Pogba was one of many left stunned by his side's collapse in their 3-0 home defeat to Tottenham on Monday, a result which leaves Mourinho under pressure after two defeats from the opening three Premier League matches of the season.
But former England and Everton boss Allardyce defended Mourinho in the wake of the latest defeat, saying they were the dominant side despite the scoreline, and instead turned attention to their World Cup-winning star.
"United fans have been waiting for a consistent level of performances from Paul Pogba for so long now," he told talkSPORT.
"Is it ever going to materialise at 25 now? He is at his peak. You have to hold your hands up and say you have not quite got what you expected on a consistent basis.
"When he plays well, he plays exceptionally well. His goals against Manchester City last season and his performance when he got moved just behind the front man was terrific, but we just haven't seen enough.
"Everybody said he had a brilliant World Cup for France and I never saw that either. I must have to go to the optician.
"When you are the leading man, you have to be leading every week. Your performances cannot dip on a week in, week out basis."
However, the France international would be welcome at Barcelona, according to defender Gerard Pique, despite Ernesto Valverde distancing the club from making a move.
Pogba has made 91 appearances for United since joining from Juventus in 2016 and remains contracted to the club until 2021.