Eight-goal Garner was forced off with a hamstring injury midway through the second half of Saturday's 2-1 home defeat by Accrington Stanley.
The 23-year-old has already missed two-and-a-half months of the season after being injured on Boxing Day.
Asked if Garner's latest setback could end his season, head coach Crainey said: “Potentially it could but it just looks like that has been the same story right through the season, not being able to get a full squad to pick from.
“But he's injured and we need to adapt and get on with it and that's definitely we are going to do.
“Ged being stretchered off doesn't look good but we dust ourselves down and we'll be fighting like mad, I guarantee that.”
With the final five games of Fleetwood's League One relegation battle being played over just 16 days this month, Crainey would welcome as many options as possibly, though goalkeeper Alex Cairns faces a suspension after his red card against Stanley.
Crainey added: “As a manager I want as many players as possible to pick from. I'll pick a team and a squad I think will win individual matches and that's what I'll do for Oxford (who visit Highbury on Friday).”
The boss could provide encouraging news about Anthony Pilkington, whose absence after half-time on Saturday was precautionary.
Crainey said: “He was feeling his hamstring a wee bit tight in the first half and I didn't want to take any chances, that's why he went off.”