Mourinho had believed the Belgian stopper sustained a serious injury when speaking after the win over Manchester City on Sunday.
But those fears have been allayed after later scans.
"Toby has a muscular injury but not as big as we initially thought," Mourinho told a press conference ahead of a Europa League meeting with Ludogorets on Thursday night.
"He will have his time to be out but not like a month, which was immediately what we thought. I'd say between two and four weeks."