Kevin Long is Portsmouth's latest defensive casualty.
The on-loan centre-half hobbled out of Saturday's home defeat by Walsall with a back injury and is now set for a return to parent club Burnley.
Boss Michael Appleton was already without Gabor Gyepes and Ashley Westwood due to ankle ailments, while loanee Paul Connolly was forced to go back to his club Leeds with a hamstring problem.
Appleton may have to hand Slovakian defender Lubomir Michalik his debut despite a lack of match fitness.
Winger Jordan Obita returns after completing the three-match ban he received for his red card against Oldham a fortnight ago.
Summer signing Gary Roberts is pushing for a place in the Swindon starting XI.
The former Huddersfield man made his Robins bow in the 2-2 draw with Carlisle at the weekend, coming on as a 52nd-minute substitution after finally recovering from calf and lower leg injuries which have hampered him for the first part of the season.
Manager Paolo Di Canio has backed Roberts' fellow new addition Andy Williams to break his Swindon duck. The striker has failed to find the net since moving from Yeovil in the summer, where he notched 17 times last season, but is set to keep his place in the Town front-line.
He could be joined by James Collins, who stepped up his recovery from injury by coming on for the last 30 minutes on Saturday.
Defender Troy Archibald-Henville is likely to start on the bench again.