Neil Harris described himself as “absolutely gutted” as Cambridge missed a chance to keep their survival chances alive.
The U’s led for a long stretch of the game but fell 2-1 at home to Leyton Orient.
A win would have left them two points behind Burton, who they play next, but defeat means they are five points from safety with only two games remaining.
“I’m so disappointed today. It’s an opportunity missed,” Harris said.
“To close that gap even to four points, or two points, is an opportunity missed and I’m not going to shy away from that.
“My job is to pick the group up when we’re back in. I need to pick myself up first, because I’m gutted today, absolutely gutted. We know what we have to do next week, and even if we win next week it might not be enough.
“One of the lads in the changing room said it just sums us up, we can’t score at one end and we concede a poor goal out of nothing at the other end.
“Sometimes when you’re at the top of the division like Orient are and they’re fighting for promotion, those opportunities go your way. When you’re at the bottom of the league like we are those opportunities don’t find the net.
“We’ve not given loads away while I’ve been here but we’ve also not scored enough and today is another one. We’re just not clinical enough.
“Next week we have to go and win at Burton, so at least there’s clarity in that situation.”
Orient’s win sees them stay in the final play-off spot in sixth.
Orient boss Richie Wellens said: “I said when I first joined this football club that if the fans are with me, and they’re with the players we’ll take you on a journey. Let’s just enjoy the journey because at the moment it’s not a bad journey, is it?
“I’ve said it all along; we were relegated by November, then we were safe by December, then by March we can’t get in the play-offs. It is what it is.
“I warned the players that in the first half I don’t want too much possession without going anywhere, they’ll be a threat from set plays and the counter attack, and we got done on the counter attack and moved the ball too slow.
“They were really disciplined, kept the wingers very narrow. You get what you deserve. Even though we had all the ball they deserved to be in front in the first half.
“In the second half we dropped Jordan Brown as a third centre back, and I thought that the angles that gave us caused them more problems. In the end we probably deserved it.”
