Heeney missed Friday night's pre-season win over GWS after feeling unwell last week, and has since been diagnosed with the energy-sapping illness less than three weeks out from the Swans' season opener against Port Adelaide.

It's a major blow for Longmire who had planned on shifting Heeney to the midfield after his sensational rise to stardom in the second half of last season.

The 20-year-old played in midfield in the Swans' first pre-season game against North Melbourne, and also starred in that role last month in the club's intra-club trial.

"He's had a fantastic pre-season to the point where he couldn't do much more," Longmire said.

"He's going to do some training next week and get back in amongst it next week. What he can do will be guided by how he's feeling ... guided by the doctors and how he recovers from it.

"Each individual case [of glandular fever] is such [that] you can't just put a broad brush on and say it's like a hamstring. We're confident he'll be fine early in the season, we're just not sure how early."

Heeney's silky skills and physicality will be missed against Port Adelaide in round one, and most likely a week later in the grand final rematch against the Western Bulldogs.

But Longmire is flushed with options when it comes to replacements in a midfield that already boasts Josh Kennedy, Luke Parker and Dan Hannebery, despite the loss of Tom Mitchell to Hawthorn.

Zak Jones has enjoyed an excellent pre-season, and has speed to burn through the centre of the ground. He's no stranger to the big stage having played in last year's AFL decider despite the sickening head knock he copped in the NEAFL grand final three weeks prior.

Longmire may also turn to George Hewett, who played up forward for most of last year but who has enjoyed moving further up the ground this year. He was one of the better performers for Sydney against the Giants, after a strong showing two weeks earlier against North Melbourne.

Then there is last year's Rising Star Callum Mills, who has been spoken about outside the club as a walk-up start in the Swans' midfield..

However, the feeling inside the club about Mills' role in 2017 is a little different with vice-captain Dane Rampe saying on Saturday that he wouldn't be leaving the Swans' backline any time soon.

Backs coach Henry Playfair, who was in charge against the Giants, also doesn't expect Mills to change positions just yet.

"He's a great half-backer - he does the job so well back there it's hard to move him," Playfair said.

"We'll constantly keep in our mind that he can go midfield and I'm sure he'll get exposure - he got exposure last year. We'll just get that balance right for the team."