Celtic manager Ronny Deila says his side will face a real battle to retain their Scottish Premiership title this season after Aberdeen leapfrogged the Hoops at the top of the table following a 2-1 win at Pittodrie.

Paul Quinn prodded home four minutes from time as the Dons recovered from going behind and having a player sent off to secure victory.

Leigh Griffiths had put Celtic in front with a 35th minute penalty before Adam Rooney levelled matters with his own second half spot-kick.

Jonny Hayes was shown a straight red for a lunge on Mikel Lustig before Quinn sealed a victory that sends Aberdeen two points clear of Celtic at the top of the table.

The Hoops claimed their fourth league title in a row last term, finishing 17 points ahead of nearest challengers Aberdeen.

However, after watching his side lose in the league for the first time since March, Deila believes Derek McInnes's side will pose a more serious threat this season.

"They are a good team and they win games. If they continue to do that then we are going to have a serious challenge here," Deila admitted.

"We are disappointed that we didn't perform better than we did but what is done is done. We have to move on and learn from it."

The Celtic manager, whose side now travel to Amsterdam for their Europa League match with Ajax on Thursday, was disappointed with his side's defending and some of referee Craig Thomson's decisions during their defeat.

"I think we played a controlled first half. We were the best team and created the most chances," Deila said.

"The second half started ok as well but we got the penalty against us, which I think was no penalty.

"And of course the end of the game was very disappointing. They get one sent off and we should have attacked and scored a second goal and instead we conceded one in an easy set play.

"It was a high ball in and there was some very poor defending."

Aberdeen have now made their best ever start to a league campaign with six consecutive victories.

The Dons could extend their lead at the top to five points if they can win their game in hand against Hamilton at Pittodrie on Tuesday. "I'm delighted and we got our just reward for that. I thought we were the better team first half and then we go behind," McInnes said.

"But the response was terrific. We had to show exactly why winning was so important to us and I thought we saw that from the first second of the second half.

"Every one of the players showed determination in abundance to make sure the game went our way."

Aberdeen's title challenge was derailed last season as they lost all four league meetings with Celtic and McInnes stressed the importance of ending that sequence.

"It was important that we saw improvement and that improvement has to come with results," the Dons boss said.

"Our performance was probably similar to a lot of games last year but no one is really bothered by that, it's results that people pass comments on.

"It was important that we got the result, rather than the performance today.

"For us, it's three points but there's no point in doing that and then not winning the game on Tuesday against Hamilton.

"We know it's an important three points but it's early on in the season and we have work to do."