By Chris Harris in Glasgow

Abou Diaby is back in the squad to face Celtic but three of his team-mates face fitness tests ahead of Tuesday night's vital Champions League qualifier.

The French midfielder missed the Premier League kick-off with a groin injury but he is in contention for a starting place at Parkhead. However Cesc Fabregas, Denilson and Bacary Sagna are all minor doubts for the first leg while Theo Walcott will not feature.

Arsène Wenger allayed fears when he described Fabregas' chances of playing as "much more" than 50 per cent but the manager still faces an anxious wait for news on the morning of the match.

"Theo Walcott is not fit but Abou Diaby is with the squad here," said Wenger at his pre-match press conference.

"From Everton we have Sagna who has a knock, we will make a decision about him tomorrow. We have some uncertainties about Fabregas and Denilson as well but we will make a decision about them tomorrow as well.

"It is more than 50 per cent that Cesc will play, much more. He has a tight hamstring, I took him off at Everton because of that. Denilson has a groin problem.

"We will play the best possible team tomorrow, the team which I think has the best chance to win the game. It is a very important game for us because we know what is at stake and we know how difficult it is to play here."

Walcott was expected to return against Celtic but could also miss Saturday's Premier League clash with Portsmouth after the back injury which ruled him out of England's friendly in Holland flared up again.

"We are not sure [if he is fit for Saturday]," said Wenger. "It is a back problem which he took with England to Holland. He had a test this morning, it was negative.

"The injury happened before [he went on international duty] and they didn't send him back from England because they thought it would be possible for him to play two days later. But it came back and he had to stop training when he had a test this morning."

Tomas Rosicky (hamstring), Samir Nasri (broken leg), Johan Djourou (knee) and Lukasz Fabianski (knee) are also sidelined for Tuesday night's game.

Copyright 2014 The Arsenal Football Club plc. Permission to use quotations from this article is granted subject to appropriate credit being given to www.arsenal.com as the source 17 Aug 2009