Manchester United fought back from a goal down to beat Bournemouth at lunchtime.
A 92nd minute winner from Marcus Rashford gave United a deserved away win.
United had conceded to an early goal from Callum Wilson before Anthony Martial equalised.
Mourinho criticised United’s defending in the first half as a disaster.
The two teams went in 1-1 at half-time with United lucky to be level.
Speaking to the club website, Mourinho said he tried to be upbeat in the dressing room during the break.
He said: “Strong, but calm words [at half time]. I started by being positive by saying that it was impossible to play worse than we did in the first half, so the second half is going to be better for sure and it was, obviously.”
He added that he was frustrated by the team continuing to start slowly, “I really cannot understand the kind of [early-match] mentality that brings me more white hair!”
United were dominant in the second half and should have won the game long before Rashford’s winner.
Substitute Ander Herrera was excellent and nearly scored, with fellow sub Rashford scoring the vital late goal from close range.
United are now up to seventh place in the table and within reach of the Champions League places.