Bruno Fernandes' 76th-minute penalty completed a comeback after the visitors went down to 10 men.
United had its worst-ever start to a league game. Poor defending helped give Forest lead 2-0 after less than four minutes. Taiwo Awoniyi scored on a counterattack after just 86 seconds, and Willy Boly quickly doubled the lead from a free kick.
Casemiro wasted a great chance to level but made amends shortly after halftime when the midfielder slotted home at the end of a clever free kick routine.
Joe Worrall was shown a straight red card for denying Fernandes a goalscoring opportunity in the 67th, and the United captain then scored the winner from the spot after Danilo was adjudged to have brought down Rashford.
It secured a second win in three games for United, after falling at Tottenham 2-0 last weekend.
“It wasn’t the perfect start for us but we kept control, kept the ball and started doing the right things," Fernandes said. “They defended well with a lot of players and it was difficult to get through, but we got the result and that’s the most important thing.”
Thousands of fans stayed in their seats after the game to stage a sit-in protest against United's owners, the American Glazer family, whose talks over a possible sale of the club are dragging on despite interested buyers.
AP soccer: https://apnews.com/hub/soccer
The Associated Press