The Gunners had hoped that the England international would be able to make his return from an abdominal problem in October but his rehabilitation was mishandled
Theo Walcott would have returned to the Arsenal squad a month ago had he not been rushed back into training.
Gunners coach Arsene Wenger had initially predicted that the winger would be out until November after he had surgery on an abdominal injury picked up on September 18.
But the Frenchman had been hopeful of including Walcott in his squad for the home match against Norwich on Saturday, October 19, which they subsequently won 4-1.
It is believed that, on the advice of Arsenal’s medical staff, the England international returned to outdoor training on the Thursday before the Canaries fixture and he was considered to be in contention for a place in the matchday squad.
But sources have revealed that Walcott was taken outside by the club’s fitness staff “a bit early” and was subsequently forced to stop running because he was experiencing too much pain in his stomach.
The setback delayed the 24-year-old's comeback and ended Arsenal’s hopes of fast-tracking him back into the team.
A second, shorter period of rehabilitation followed and Walcott returned to outdoor training in the build-up to the Manchester United match, which the Gunners lost 1-0 before the international break.
But Wenger and his staff were only prepared to consider the Englishman for the Old Trafford trip if they had suffered a rash of injuries in the wake of the heavyweight clashes against Liverpool and Borussia Dortmund.
It was felt Walcott needed at least 10 days of running before he could feature in a first-team match and he ultimately did not travel with the squad.
Walcott has been training fully during the international break and Wenger revealed on Thursday afternoon that the player is “completely fit again” and “will be back for Saturday in the squad”, when the Gunners host Southampton in the Premier League.
The attacker's return is a major boost to Wenger, whose options in the wide areas have been restricted this season by serious injuries to regulars, including Santi Cazorla and absent duo Lukas Podolski and Alex Oxlade-Chamberlain.
“Theo has different qualities to other players and I’m glad he’s back,” the Arsenal boss enthused.
“He is one of the players who goes behind [the defence] without the ball. Strikers who get behind the defenders is a very important quality and Theo has that quality.
"Of course you miss a player like Theo Walcott. Theo had a good start to the season, he scored goals, he provided chances and had assists. He can always give you a different option in the game and that is of course vital.”
Despite his manager’s fulsome praise, Walcott’s productivity levels have dropped since January, the month he ended speculation about his future by signing a new contract.
He has played just 22 matches for Arsenal in the last 10 months, scoring four goals.