McDermott has a groin strain and the midfielder was due to see a specialist on Thursday night.
Defender Chris Hussey is also out as he is set to undergo a hernia operation while Timi Max Elsnik is still around 10 days away from recovering from a hamstring injury.
Dion Conroy, meanwhile, remains a long-term absentee after injuring his anterior cruciate ligament.
Haydn Hollis could make his Forest Green debut at the County Ground after arriving on an 18-month deal from Notts County this week.
Midfielder Charlie Cooper will miss the game as he continues his three-match ban for a red card received in the recent clash with Wycombe.
New signings Reuben Reid, Gavin Gunning and Farrend Rawson are expected to play after they were cup-tied for the midweek Checkatrade Trophy game against Yeovil.
Lee Collins and Christian Doidge should also come back into the squad for the trip, but Dale Bennett is still struggling with the ankle injury he sustained before Christmas.
Southampton manager Mark Hughes was delighted to see his new side reach the FA Cup semi-finals but also warned that the Premier League remains a concern.
Borussia Dortmund winger Andre Schurrle was delighted with the team's hard-fought 1-0 Bundesliga victory over Hannover 96 on Sunday afternoon.
Chelsea defender Andreas Christensen has revealed that he sought advice from club legend John Terry after making a few costly mistakes recently.