Sydney Swans coach John Longmire says there's no cause for alarm after Callum Mills was ruled out of Saturday's clash with Richmond at the SCG.

Mills will miss his second game of the season after developing a sore calf which forced him to sit out Thursday's main training session.

Longmire flirted with the idea of playing the 19-year-old, who's starred across half-back in his first year of AFL football, but the risk of straining the muscle or doing any more damage was too great.

"He was touch and go whether he'd be right to play," Longmire revealed in this week's Centrum Team Selection.

"It would have been a bit of a risk of doing his calf if we played him on the weekend, and that's just too big of a risk to take.

"He's a very good player for us so we just decided not to take that risk."

Mills has averaged just under 20 disposals per game across his 20 appearances this season, coming off an impressive display against North Melbourne in Hobart.

He's one of the favourites to win this year's AFL Rising Star Award.

Mills was one of two changes for Saturday's twilight fixture against the Tigers, with Kieren Jack and Harry Marsh replacing Mills and Dean Towers (omitted) in the selected side.

Towers, Jordan Foote and Toby Nankervis have been named as emergencies.

To view the full squad, click here.