The 23-year-old has not featured during pre-season with City boss Nigel Pearson unsure when his star forward will be ready for action.
"I can't put a timescale on Matty's return. I'm not an expert on how the body's immune system reacts to infection. He'll be ready when he's ready," Pearson told the Mercury.
"He's on antibiotics and getting better, but I don't know how long we are going to have to wait to get him back. But I'd rather wait that little bit longer and him to be right."