Robbie Neilson said Dons were nowhere near good enough as they handed Portsmouth a simple 2-0 win at Fratton Park on Saturday.
Oliver Hawkins was gifted two simple goals after defensive errors from Ethan Ebanks-Landell and Scott Golbourne to effectively wrap the game up by half time.
Dons didn't register a shot on target during the 90 minutes, while Portsmouth themselves hardly tested Lee Nicholls in the Dons goal in a game void of much quality, but Neilson admitted his side were poor and deserved nothing from the south coast.
"The quality was miles off where it needed to be, but we have to take it on the chin," he said. "We had opportunities to pass the ball better, but we didn't. It is up to everyone to improve. Today, we took a backwards step.
"People make mistakes - that's football. Yes it had a big baring on the game, but it came down to the performance on the whole and it wasn't good enough.
"We started alright, the quality into Osman was good but we didn't link it well enough and the quality in the attacking area was really, really poor. It allowed Portsmouth onto the front foot, we gave away a goal and it spiralled from there.
"We had words at half time, but it just wasn't good enough. It's a fantastic place to come and play, but we didn't turn up after the first 15 minutes.
"The second goal is always key. Today was just a bad day all round. The big thing about this league is being resiliant. We have to go again on Tuesday and make sure we put in a better performance and take three points."
Dons remain 14th in the table despite their second defeat in as many games, but with two more games this week, at home to Walsall on Tuesday and Oldham on Saturday, Neilson is keen to put the result behind him.
He added: "It's hard to put together a long, sustained winning streak. We managed to do it but the last couple of weeks have been disappointing. But we've got the chance to get back on the horse on Tuesday night."