A loan deal was announced on Monday, but Mason has been told to return to south Wales after strikers Rudy Gestede and Peter Odemwingie were injured over the weekend.
The deal could be revived next week, with Mason seeking regular first-team football having failed to make an appearance in the Premier League this season.
"Gestede and Odemwingie both took wee knocks and our thought was that we had to be sure both are okay," said Mackay.
"Neither is serious, but I felt it was right to bring Joe back. Hopefully, though, the loan move could be completed in a week or so.
"It's simply a case of being certain exactly how Rudy and Peter are."
Mackay also revealed there has been no row between him and Wanderers manager Dougie Freedman following the decision.
He added: "I talked to Dougie and apologised to him. He understands that we have a couple of minor injuries and just want those checked out.
"We are hopeful something will be done about the loan next week."