Fryatt, 23, has not featured during pre-season with the Foxes and boss Nigel Pearson is unsure when his star frontman will be ready for action.
He told The Mercury: "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. 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."
The Preston North End manager Alan Irvine has played down rumours linking his club with a move for former Germany midfielder Dietmar Hamann.
The 35-year-old is a free agent after leaving Manchester City, but Irvine says that a deal is highly unlikely.
He told the club's official website: "We were linked with Didi Hamann and that was quite simply a phone call from an agent.
"We are working away trying to see what we can do, but it has to be within our means."
West Bromwich Albion have confirmed that midfielder James Morrison is a major doubt for the start of the new season due to a heel injury.
The Scotland international, 23, missed to end of last season and has not yet fully recovered from an operation.
Carlisle United have hit out at Watford over the delay in the tribunal to settle a fee for striker Danny Graham's move to Watford.
A Football League tribunal should have met this week, but the Hornets have asked for a delay as football operations manager Iain Moody is away on the club's pre-season tour of Spain.