The Daily Mail says the Middlesbrough midfielder faces up to six months out injured.
The England international is set to undergo surgery on his foot on Friday and he has been warned he could be out until Christmas, putting his £10million summer move to Liverpool or Tottenham in serious doubt.
"We will not be able to assess the full extent of Stewart's injury until his bruising and swelling go down," said Boro's head of medical Grant Downie.
"We are still waiting for further tests and investigations to find out the best way to manage the problem. Stewart has already seen a leading orthopaedic expert in London and is in the best possible hands."