Kay had been due to miss the Sky Bet League Two fixture through suspension, but he now requires hernia surgery and will be out for a month.
Top scorer and captain Tom Pope is expected to start, despite Valiants’ boss Neil Aspin revealing the striker is missing training as he nurses an unspecified injury.
Marcus Harness, Tyrone Barnett and David Worrall all face checks after picking up knocks in last week’s 3-2 defeat at Swindon.
Newport midfielder Joss Labadie starts a two-game ban after picking up his 10th yellow card of the season.
Defender Scot Bennett (hamstring) is still out and Mark O’Brien continues to deputise.
Paul Hayes is available after recovering from a groin strain.
Fellow striker Marlon Jackson is also in contention after returning from a long-term hamstring injury and coming off the bench in the last two matches.
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.