Stoke manager Mark Hughes expects to be without Brek Shea for "a number of weeks" after the United States winger was injured during the 2-0 friendly win over Philadelphia Union.
Shea impressed during his first appearance under Hughes, setting up the opener for Michael Kightly before adding the second himself.
But his involvement was ended early by a challenge from Matt Kassel, with the club initially fearing the 23-year-old has suffered damage to the medial ligament in his left knee.
Shea will undergo a scan on the injury, but Hughes is expecting a lengthy lay-off.
The Welshman told Stoke City Player: "He will go for a scan tomorrow, but we know that we will definitely be without him for a number of weeks. It looks fairly serious but fingers crossed it isn't any more serious than we already know it is."
Hughes was also critical of Kassel's challenge.
He said: "I really have no idea what the lad was thinking about going in for the challenge like that.
"Maybe he was trying to make a name for himself - I have no idea. But whatever he was thinking it was totally wrong."