The Red Devils have already conceded 18 times in the Premier League this season, the worst record in the top half.
On this occasion, United went 1-0 down to Bournemouth in the 11th minute after we failed to get rid of the ball after a set piece, giving Callum Wilson the chance to put his side in front.
Mourinho was unhappy with how the first half went, branding it a "disaster", although he praised his side's spirit to make a comeback in the second 45 minutes.
He was then taken off in the 78th minute.
What Mourinho described as a "disastrous" first half performance was backed up by an improved display after the break, yet the United manager didn't try to hide his annoyance as he spoke to BT Sport after the game.
Starting XI: Begovic (7); Francis (6), S. Cook (7), Ake (8), Smith (6); Stanislas (7), L. Cook (6), Lerma (6), Fraser (7); Brooks (7); Wilson (8). However, Rashford made amends in the final seconds when he turned in Paul Pogba's cross from close-range.More news: Trump says Democrats would ruin Florida, his second home
When asked what two positive results could do for United's season, Smalling said: "I think that it would galvanise us".
"The approach [last season] was much more defensive and with a much more defensive approach you can hide better your weaknesses", Mourinho said.
The 27-year-old is certainly the best player we have at United and was brilliant in our 2-1 win over Bournemouth on Saturday.
Mourinho's second half changes shifted he course of the game as Ander Herrera made an impact from the bench and Rashford scored the victor after he replaced Juan Mata after 56 minutes.
The Reds were in for a tricky afternoon given Eddie Howe men's fine form this season, but the game ended with United coming back from a goal down to snatch all three points.
Chris Smalling had a hard game against Bournemouth, as the centre back looked shaky for large spells of the game. Absolutely very bad. And not just my defenders, but the team.
Bournemouth's next game is a trip to Newcastle, as they look to bounce back following the manner of this defeat.