Arsene Wenger admits his Samir Nasri gamble 'backfired' after the influential midfielder picked up a hamstring injury against Huddersfield.
The French playmaker is now a serious doubt for the Gunners' crucial UEFA Champions League clash against Barcelona after sustaining the knock in Sunday's 2-1 FA Cup fourth round win over League One Huddersfield.
Wenger made sweeping changes from the side which had booked a place at Wembley with their midweek Carling Cup semi-final second leg triumph over Ipswich.
However, due to concerns over the fitness of Tomas Rosicky, who has been suffering with a virus, the French boss chose to put Nasri in his starting XI.
But Wenger has been left rueing that decision after the 23-year-old limped off with a hamstring problem in the first half.
The manager said: "Nasri looks to have a serious hamstring injury. Usually it is three weeks.
"We have to see if it is a grade 1, it will be two weeks, so we will see.
"I have now regrets because the plan was to play Rosicky, but he was still too weak because he was sick, so I took the gamble on Nasri.
"It backfired, but he could have come on and that happened as well. It is very difficult to predict.
"We are at the stage of the season where we play so many games that to lose bodies is very difficult for us. But Samir is a big blow."
Wenger admits Huddersfield gave his battle-weary players a great test of character, adding: "It was tough, tough, tough and had all the ingredients of a typical cup tie.
"I think we played a bit below par, and Huddersfield are a good team - they were efficient, dangerous on set-pieces and crosses.
"In the end, though, we had the quality to make a difference, but we play so many games at the moment that of course sometimes we are not as sharp as you want to be."
Wenger felt the red card handed to Sebastien Squillaci, who was sent off late in the first half for a challenge on Jack Hunt, was "harsh".
The Arsenal manager said: "You can give it and not give it. It was a foul, but it didn't look to me that it was an obvious goalscoring opportunity.
"Is he the last man? That I am not sure as well. I feared the red card and unfortunately he gave what I feared."
Cesc Fabregas, who was sent on in the second half and notched the winner from the penalty spot, appeared to gesture to the referee for a red card as further punishment at the spot-kick.
Wenger, though, maintained: "Cesc thought it was a red card. It is for the referee to make the decision. He made the right decision in my opinion.
"In the box, personally I am more lenient than outside."