Brek Shea is a doubt for the start of the Premier League season after sustaining a knee injury during Stoke's 2-0 win over Philadelphia Union.
He said: "Obviously I'm pretty upset right now. I was excited to get back to Stoke and get ready for the season. Hopefully it's not as bad as it feels."
Manager Mark Hughes was fuming about the incident, saying, "It was a nasty injury. It looks like some damage to the ligaments in his knee. He's going to be scanned. I thought it was a very, very poor challenge by the boy. What the hell was he was thinking of, I have no idea.
"It's a friendly game. You need to protect your fellow professional. And I don't think the guy did that. I don't know who he was. Maybe he was trying to make a name for himself, I don't know. He went the wrong way about it."
However, Shea defended the challenge made by Kassel.
He said: "He was going for the ball. I tried to put my body in front of his and it's soccer. Tackles happen - good and bad. I've done worse to other people. I'm not mad at all.
"When I was walking off, he apologized."