Manager Arsene Wenger admits Arsenal do not know how long Tomas Rosicky will be sidelined for following knee surgery, but said Jack Wilshere was on schedule with his recovery.
Rosicky, who agreed a contract extension at the end of last season, has been struggling with a knee problem since June following international duty with Czech Republic, and is set for another lengthy spell of rehabilitation.
Wenger suggested the club were reluctant to place any specific time frame on the recovery of the 34-year-old, who has endured a string of fitness issues over his Arsenal career.
"Tomas has had surgery on his knee. His agent said he will be out for one or two months, but it will be longer," Wenger told the club's official website. "How long exactly, we don't know yet."
The prognosis on Wilshere, however, is more positive.
The England international had impressed during the preseason trip to Singapore and in the Emirates Cup, but suffered a hairline fracture in his ankle on the eve of the FA Community Shield against Chelsea at Wembley.
Wilshere is said to have been recuperating in Dubai to help aid his natural healing process.
Wenger added: "We said Jack would be out for four weeks, and he's in the second week now so he's on schedule."
Arsenal should have full-back Hector Bellerin available again for the trip to Crystal Palace on Sunday after sitting out the 2-0 home defeat by West Ham, but while Wenger says forward Danny Welbeck is "making good progress" from a knee problem, he is not expected to feature.
A surprise inclusion on the bench against West Ham was Alexis Sanchez, who had just returned to training following an extended break after helping Chile win the Copa America.
Wenger said the forward was keen to get back into action again.
He said: "Alexis Sanchez is fit, he's been working hard and is ready for the weekend."