Manager Arsene Wenger admits the absence of striker Danny Welbeck is a concern for Arsenal.
The Englishman is yet to feature for the Gunners this season after suffering an injury in April.
Back then, the Frenchman expected the 24-year-old to be ready for pre-season but he is yet to participate in full training and will miss Sunday's away game against Crystal Palace.
"It is much slower than expected," Wenger told reporters on Friday morning.
"It was a bone bruising at the start. You think that it will be quite quick.
"Danny is an important player for us and the fact that he is not available is a bit of a concern.
"They have done all kinds of scans that today are very sophisticated and some of them I don't understand at all, but they tell me no [operation is needed].
"[At the moment] he is [training] in the gym."