Darren Ferguson urged Peterborough not to lose focus after they failed to clinch automatic promotion in a 2-2 draw against Doncaster.
The second-placed side needed victory against Rovers to earn a rise into the Championship.
All was going to plan when Joe Ward fired Posh into an early advantage, which was doubled when Sammie Szmodics converted Jack Taylor’s cut-back midway through the opening period.
But as fans gathered around the stadium, Doncaster ensured the promotion party turned flat.
Fejiri Okenabirhie halved Rovers’ deficit seven minutes before the break and substitute Taylor Richards completed the comeback just before the hour after Posh man Ward had twice been denied by the crossbar.
It means Posh must wait to seal a ninth promotion in their English Football League history.
Ferguson admitted: “After 25 minutes we were thinking it could be a really enjoyable night for us.
“We were outstanding in that part of the game, but then got a bit sloppy against a good team.
“We all know things can change quickly in football and Doncaster’s first goal definitely lifted them. It gave them hope and affected us too much. That was the moment on which the game changed.
“We then started the second half poorly and gave away another goal before their keeper made three outstanding saves to keep them level.
“We are disappointed because we know we had a great opportunity tonight.
“I now have to ensure the players regroup and keep focused. We have come too far to lose our belief and we know we’re still in a good position.
“All our thoughts are now on the game against Lincoln on Saturday and trying to win it.”
Rovers player-boss Andy Butler, who was robbed of four players following a positive Covid-19 case, said: “We didn’t start as well as we should have, but after going 2-0 behind we played some good football.
“It’s a pleasing performance and shows we can match a top side, but just a shame that it has come too late in the season.
“I told the players that we want to finish on a high and carry it on into next season.
“We could hear the Peterborough fans outside the ground, but I listened again at the end and they seemed to have gone. The fireworks had stopped as well!
“Peterborough are a good side who fully deserve to be where they are and I’m sure they will get promoted.
“We have delayed the party and that’s credit to the players for not letting Peterborough be able to celebrate in front of us.
“But we know we should be in the play-offs ourselves and being totally honest we’ve let ourselves down.”