Dons' failure to react to a resurgent Grimsby Town side at Blundell Park on Saturday resulted in a 3-1 defeat and FA Cup knockout, according Robbie Simpson.
Despite leading at the break, and being in relative control of the game, Dons threw away their advantage just five minutes after the restart when Elliot Embleton scored a wonderful strike from the edge of the penalty area. Goals from Wes Thomas and Charles Vernam followed as Grimsby ran out comfortable winners in the end.
While the saving grace is that Dons do not lose any ground in League 2, Simspon said it is of little consolation to the players, who needed to do better when they fell behind.
"We weren't thinking about the league today, we wanted to progress in the FA Cup," said the striker. "We're really disappointed with the result, obviously, and with the performance in the second half.
"It's the first time we've gone behind and haven't responded and stayed strong. To concede another goal, and another without getting control of the game, we're really disappointed and we have to learn from it.
"I thought we were good in the first half, with the wind and slightly downhill. Going in at half time, we were in a good position, which makes the second half even more disappointing. We've been in windy conditions and done really well, like last week, but that goal lifted the atmosphere.
"Credit to Grimsby, they played the conditions and not like a team at the wrong end of League 2. We're still getting to know each other. Although it's disappointing, we have to take this loss as a positive, so when it happens again, we are better equipped to deal with it."
Simpson said while manager Paul Tisdale was not one for reading the team the riot act, he said the players know they cannot afford to let their performances slip so significantly, irrespective of the opposition.
He said: "Tis is very poignant in his messages. When we conceded the third, you don't see him ranting and raving, throwing his arms about on the sidelines. We lost our discipline and shape in the second half. We cannot do that. Whether it's against Grimsby or Manchester United, we can't let that happen no matter what the scoreline, but especially not when we go behind."