Teenage defender Aidan White was not in the squad for Saturday's home defeat to Huddersfield in the West Yorkshire derby, but could be recalled to the squad.
After the disappointing defeat to Huddersfield, McAllister was keen for his side to push forward and focus on the cup tie.
"We move on now," McAllister told the club's official website.
"We'll look at everyone on Sunday when we warm-up and start thinking about Monday.
"Northampton is a big game, especially coming after a defeat, but that's our focus now."
In the first meeting at Elland Road, Andy Robinson's penalty cancelled out Scott McGleish's opener in a 1-1 draw.
Northampton will welcome back Giles Coke from suspension after the midfielder was sent-off in the 1-1 draw at Elland Road.
However, captain Chris Doig is ruled out with a hamstring injury after lasting just 11 minutes of the last meeting with the Yorkshire outfit.
Striker Adebayo Akinfenwa (calf) and midfielder Adbul Osman (knee) are also unavailable for selection.
Loan-signing Kyle Walker made his debut against Oldham last time out, but is ruled out of the replay as he was not with the club at the time of the original game.
Sporting Life