The 23-year-old flew back to his native Melbourne claiming he was "depressed" and leaving Celtic in no doubt that he was giving up football just days after his pounds 3m transfer from Croatia Zagreb.
McCann also indicated that he will be withholding full payment from Croatia Zagreb until the matter is fully resolved.
"The club is now most concerned about certain aspects of the transaction regarding Marko Viduka," McCann said. "We will be seeking clarification over the next few days from various parties, including Croatia Zagreb."
As to Viduka's claim that he was quitting because he was "not in the right state of mind to play football", McCann said: "It is a sudden problem. There was no indication from the player, Croatia Zagreb or IMG [Viduka's agents] of any psychological problem."
The problem only came to light when the manager, Josef Venglos, and club doctor, Jack Mulhearn, visited Viduka at his hotel on Saturday. "We offered medical counselling, and to bring his parents over from Australia as well as giving him an extended Christmas break but Marko rejected all these offers. He said he didn't want to play football," McCann said.
"Marko said he knew he was giving up a wonderful opportunity and that his decision would wreck his career.
"There are question marks surrounding the fee and where it is going. There is a lot more to this than Marko suddenly saying he has psychological problems."