McClean was hurt in a heavy challenge during the first half of the Republic of Ireland's 2-0 friendly victory over Poland in Dublin on Wednesday evening. However, he played on after treatment and it was only later that the extent of the problem became clear.
Black Cats boss Martin O'Neill said: "We have a problem with James McClean, who picked up an injury during the course of the game."
He added: "He hurt himself in the first half, but continued in the second half and came off after, I think, 80 minutes in the match and arrived here, where his ankle is particularly swollen.
"That's causing us some concern at the moment."