The forward, 24, who had earlier scored his 12th League One goal of the campaign, is a major doubt for this weekend's FA Cup second round clash at Blue Square Premier League leaders Histon Town.
"Jermaine's got a hamstring problem," confirmed United boss Gary McAllister. "We'll need to wait to see the degree of the problem there."
Of his side's display at Sixfields, he added: "The result's very disappointing. They got in our faces in the first half and we didn't get going at all.
"In the second half we responded well and got on the front foot, but we've conceded a bad goal again.
"Their first goal was a good one, but their second was down to us not picking up players in a crucial area."