The young defender has been training with the Gills for a week following his loan move from Premier League Crystal Palace.
Inniss has made just two previous appearances in the Football League, during a loan spell at Cheltenham Town.
But the highly-rated central defender arrives to put pressure on current centre-back pairing Leon Legge and captain Adam Barrett.
Barrett was dropped for Gills' recent midweek trip to Stevenage, in preference to Callum Davies, but immediately reinstated in the game at Carlisle days later.
Gills boss Peter Taylor may also bring Elliott Hewitt straight back into the team, as he is available again following a suspension. The on-loan Ipswich player was sent off at Stevenage and replaced at right-back by Michael Harriman.
Chris Whelpdale (ankle) is expected to be available, as is Connor Smith, who picked up a knock during the win Carlisle.
United, who were beaten by the Gills at Bramall Lane earlier this season, head to Priestfield buoyed by their weekend FA Cup win over Nottingham Forest. They had already knocked Premier League Fulham out of the competition.
"They are a good team and we know it's a hard game," said Gills boss Peter Taylor.
"They had a great result on Sunday but if we start better than them and make them a bit nervous then the weekend’s match goes out of the window.
“We need to make sure we are extremely positive for the game.
"Hopefully they find Priestfield a bit different to the two stadiums they have just played at and we can get a result."
United sit in the League 1 relegation zone while the Gills head into the match sitting 12th.
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