Middlesbrough central defender Chris Riggott could be sidelined for up to eight weeks after damaging ankle ligaments in a training ground accident.
The injury could not have been worse timed for Riggott, who faces strong competition to hold on to his place in the team after Boro signed Jonathan Woodgate and Robert Huth last week.
Manager Gareth Southgate told Boro's official website: "It's a real blow for Chris because it's a big season for him and he would not have wanted it to start in this way.
"He's matured as a player enormously over the last couple of years and it's a shame for him that he should pick up an injury like this at this point.
"We will get on with it, we've just strengthened the centre of defence so we're better off than we would have been and as always in these situations, it creates an opportunity for others."
Huth is still recovering from an ankle injury, so Woodgate is likely to be partnered by Emanuel Pogatetz at Arsenal on Saturday.