Midfielder Karwan has been on trial for the past week and featured in a behind-closed-doors 1-1 friendly against Doncaster on Wednesday. However, John Carver, decided against a move.
"We've had a look at him and we've assessed it, and as it stands at the moment he is not for us," said the caretaker boss.
But United will continue to keep an eye on Woking and England semi-professional international kepper Shwan Jalal.
"The goalkeeper has gone back to Woking, but we're keeping an eye on the situation," added Carver.
"We're going to get Hodgy [goalkeeping coach Martin Hodge] to follow up on that one because Jalal wasn't allowed to play in the game we had this week."