SYDNEY has suffered a blow, with young defender Lewis Melican re-injuring his hamstring.

The news is a massive blow for Melican, who hasn't played senior footy since first straining his hamstring against Greater Western Sydney at the SCG in round three.

Swans coach John Longmire told radio 3AW that Melican suffered another setback at training during the week.

"He has done it twice on each side now which is not great," he said.

"That is disappointing for both him and for us because we rate the kid as a player."

It's the third time he has had a setback in his recovery since he went down.

The 21-year-old played in the NEAFL last week on limited minutes and was set to increase his game time against the Giants' reserves side in Wollongong on Saturday, with a look at possibly being available for senior selection in the next fortnight.

The Swans have gone with two key defenders for most of the season in Melican's absence, but Aliir Aliir has been used to provide support for Dane Rampe and Heath Grundy for the past two matches.

With another likely month on the sidelines at the least, Melican's chances of getting back into the senior side in 2018 look to appear slim.

The former rookie was a revelation last season, playing 17 games after debuting in round five and became a reliable backman for the Swans with his uncompromising style.