Striker Gray began on the bench against Ipswich on Saturday having been unsettled by interest from Hull, but he came on to score the first of two stoppage-time goals as the Bees secured a 2-2 draw.
Danish defender Andreas Bjelland was also among the substitutes having only recently recovered from a groin injury so he could get some minutes under his belt.
Marcos Tebar is out with a hamstring injury and Josh McEachran has a broken foot while Sam Saunders and Lewis Macleod are still short of match-fitness.
Oxford boss Michael Appleton will make “one or two” changes to his side following Saturday’s draw with Crawley.
League Two games at Luton and at home to Notts County follow within seven days and Appleton is determined to keep his relatively small squad fresh.
Striker Patrick Hoban could step in if Appleton opts to rest summer signing Ryan Taylor, who was nursing a knock before the Crawley game and was substituted in the second half.
Midfielders Josh Ruffels and Callum O’Dowda will be among those hoping to step up from the bench after featuring regularly in the side last season.