Sydney Swans coach John Longmire has cautioned against putting an exact time frame on the return of emerging star Isaac Heeney.

Heeney will likely miss the start of the home and away season after being diagnosed with glandular fever.

Heeney didn't play in the Swans’ JLT Community Series win over Greater Western Sydney last Friday night with a suspected virus.

The Swans confirmed on Tuesday morning Heeney had since been found to be suffering from glandular fever and would be sidelined for at least several weeks. 

Longmire said that Heeney took the news hard, after initial tests last week failed to find a cause for the 20-year-old's sudden illness.

"He's pretty flat because he's a massive competitor who loves training and playing, so it's the total opposite of what he wants to be doing," he said.

"He'd done every session of the pre-season and he was in red hot form.

"It's disappointing for him and us, because he's an important player for us obviously, but at least we know what we're dealing with. 

"He'll rest up, and we're confident that with the quality training he had under his belt already, that he won't take long to get back to that level early in the season.

"When it is we're not really sure yet, he'll get to doing a bit of training next week, but we won't push him too hard because we don't want to set him back either."

The loss of Heeney, who had been a star of the pre-season after moving into a midfield role, adds to the news former co-captain Jarrad McVeigh (calf), and forwards Tom Papley (knee) and Gary Rohan (back) will miss the round one clash.