The USA international limped from the PPL Park pitch prior to the half-hour mark in Stoke's 2-0 friendly win, in which Shea scored, after Kassel brought him down in a strong challenge.
Shea has sustained an injury to his left knee and although the severity is unknown, Kassel was very sorry for the part he played.
"Obviously I didn't mean to do that," Kassel said. "I'm friends with the kid.
"It was completely just going for the ball. Unfortunately things happen and tonight he unfortunately got hurt.
"There was no intent at all," he added. "I wish him all the best and I apologize for the tackle."
Shea admitted that he held no ill-feeling towards 23-year old Kassel: "I'm not mad at all.
"When I was walking off, he apologized. And I said, '[Expletive] happens.'"