London - Arsenal striker Alexandre Lacazette will be sidelined until October after failing to shake off a persistent ankle injury.
Lacazette had been playing with pain from the problem for several weeks and the France international has now been told to rest in the hope it will allow time for the problem to heal.
"After assessments to an ongoing ankle issue, we have advised that Alex will need to rest and strengthen his ankle to regain full fitness," an Arsenal statement said on Friday.
"Alex has been playing through this injury for several weeks. He is likely to be back in action in October."
Lacazette has scored two goals in three appearances this season, including one in the 2-2 draw with north London rivals Tottenham in his last game before the international break.
The 28-year-old was replaced in the 67th minute of that match and won't be back in a Gunners shirt for several weeks.
His absence will be a big blow for Arsenal manager Unai Emery during a hectic period that includes Premier League games against Watford, Aston Villa and Manchester United, as well as the start of their Europa League and League Cup campaigns.
Pierre-Emerick Aubameyang will fill the central striker's role in Lacazette's absence, with the Gabon star having already scored three times this season to help the Gunners into fifth place.
While Lacazette is sidelined, Emery insists Arsenal's club-record signing Nicolas Pepe will be ready to face Watford on Sunday after confusion over his fitness during the international break.
The 23-year-old played against Tottenham before the Ivory Coast FA reported he had contacted them to say he had suffered an injury.
That claim was refuted by the Gunners and asked if Pepe was fit, Emery replied: "Yes, yes, he is training well and he is ready for Sunday."