The 'Red Devils' were 2-0 down within 20 minutes at St Mary's before Marcus Rashford, 21, spearheaded a fight back, assisting both Romelu Lukaku and Ander Herrera to level the scores
After the game, the Portuguese coach spoke of his side's lack of "mad dogs" as the reason for another disappointing result, though did heap praise on one player.
Mourinho said: "I would say Marcus Rashford was a mad dog, until he was - let's say - very, very tired and with [a few] little problems.
"It wasn't fatigue, he was asking to come out. Some player with that kind of injury they don't play for two weeks, but Marcus I'm pretty sure he will be fine for the next one.
"It is one kick here, one kick there, one fall here, one fall there, fatigue, lots of running, lots of movement - 75 minutes and he was done."