The Red Devils have seen their four-game winning run in the top flight come to an end after they lost 2-1 against Fulham at Craven Cottage on Saturday afternoon.
Following a cagey first half, Fulham were the dominant force after the interval and deservedly broke the deadlock on the 65-minute mark courtesy of a ferocious first-time finish from Calvin Bassey inside the penalty box.
Groans echoed around Old Trafford as Man United struggled to carve out clear-cut chances, until Harry Maguire - who had previously headed a glorious chance over from just a few yards out - tapped home an 89th-minute equaliser which seemed to have rescued a point for Ten Hag's side.
However, it was Fulham who had the last laugh as they burst away on yet another counter-attack that resulted in Adama Traore setting up Alex Iwobi to nestle a superb right-footed finish into the bottom corner.
Man United's 16-game unbeaten streak against Fulham in the top flight has come to an end, and the defeat comes as a huge blow for their Champions League aspirations as they remain sixth in the table and have slipped eight points behind fourth-placed Aston Villa, who beat Nottingham Forest 4-2 on home soil.
The Red Devils ultimately lacked a cutting edge in the final third and were missing the presence of injured striker Rasmus Hojlund, who had previously scored seven goals in his last six matches.
Hojlund was one of seven Man United players absent due to injury, while Casemiro was forced off with a head injury not long after Fulham opened the scoring.
Ten Hag believes the "big loss" of Casemiro played into Fulham's hands and he has reiterated that the club's troubles with injuries throughout this season has prevented his side from consistently performing at their best.
Speaking at a post-match press conference, the Dutchman said: "It's the truth [that injuries impact performances]. We don't have a left back (with both Luke Shaw and Tyrell Malacia ruled out). Casemiro is out. It's a big loss for us. [Lisandro] Martinez. But we have a group and we could have, we should have won today.
"We have to do without those players and we have to recover from the setbacks, look forward and prepare the next game."
Asked if Man United can close the eight-point gap to Aston Villa in fourth, Ten Hag added: "I said from January onwards, all games are finals and now we want many finals. Now we lost one final."
Despite the defeat, Ten Hag was pleased with the "great character" shown by his players against Fulham and was full of praise for their recent form.
"We have done very well lately, now we have some setbacks. Today we could have won this game, we should have won this game because the team showed great character.
"I think we made slow starts in both halves. Casemiro was a big loss in the second half, then we lost some stability and we conceded (the opener) in that moment.
"The goal was very avoidable, then we fought ourselves back and I have to credit the team as they showed character. We went for the win, we had our chances, but let the match slip away."
Man United will now briefly shift their focus away from the Premier League and onto the FA Cup as they prepare to face Nottingham Forest in a fifth-round tie at the City Ground on Wednesday.