Juan Mata jumped to the defence of Manchester United goalkeeper David de Gea after his mistake enabled Chelsea to claim a 1-1 draw at Old Trafford.
De Gea allowed a routine Antonio Rudiger shot to slip from his grasp, giving Marcos Alonso a simple finish just before half-time of the Premier League showdown.
United, which started strongly and led through Mata's 11th-minute goal, was unable to recover from the mistake and remain two points behind fourth-place Chelsea with only two games remaining for the sides in the top-four race.
De Gea has now made three errors leading to goals in his past four appearances, as many as in his previous 123 matches combined for United, and calls for Ole Gunnar Solskjaer to drop him are likely to increase.
Mata, though, is adamant his compatriot's position should not be questioned given his superb form in recent years.
"David is unquestionable for me," he told Sky Sports. "His level has been fantastic for this club. He's been the best player over the last years; the amount of points he has saved us is incredible.
"We all have good and bad moments over our careers. For me, the important thing is how to react, and he's determined to react positively, to do what he does best, which is be the best. I have 100 per cent confidence in him - we all have.
"Today, he is not happy, we are not happy, but tomorrow he will come to training with the best attitude as always. His level, for me, is unquestionable.
"What I can do is give him my honest consideration about the situation. He is obviously disappointed with that goal, but I honestly believe he is one of the best in the world and there is no question about that.
"Everyone makes mistakes, but if you're a goalkeeper, when you make a mistake, it's probably a goal. It's difficult for them. But he is strong - he will save many goals in the next games. He's been the best for us and he has my full confidence, and the team, the manager, everyone who loves Man United."
United will likely need to beat Huddersfield Town and Cardiff City in its final two games of the season and hope for favours elsewhere if it is to qualify for the Champions League.
"We're disappointed," Mata said. "We needed to win. We did a good first half, we scored, we had other good chances.
"It's difficult when you have the top four so close, and then you have done an incredible job over the last months to get there. We'll try until the end but obviously we needed to win today because Chelsea are fighting with us for that position.
"That's how we should start every game, especially at home. We need to do what we did today, which is put the team in trouble, press, have chances, bring the crowd with us, and I think we did very well today in the first half especially.
"We needed three points, especially because it was Chelsea, a very good team, fighting with us for the top four. But we need to move on and try to finish the season the best we can."