Arsenal could be without winger Alexis Sanchez for no less than six weeks.
The Sun reports that 27-year-old took a muscle injury in training with the national team of Chile.
The striker is expected to undergo a scan to understand the full extent of the injury.
Arsenal hope that the injury is not serious and he will be ready for the match against United next week.
However, there are fears in Arsenal that Sanchez could miss up to six weeks.