Kevin Keegan said: "It is a real disappointment. We thought he had a problem but he came back and everything seemed fine.
"But there is obviously something now that is now quite right and in a couple of weeks he may go back to America."
After the departure of Carlo Nash to Middlesbrough in the summer, City are now low on experienced goalkeeping cover.
"We are down to two keepers now and if, God forbid, anything happened then I will have to ask for special dispensation to bring someone in," said the City boss.
"I don't know if we would be allowed it for Groclin for example. We are somewhat down to the bones for that game. I will be thinking about the situation and act if needs be."