England wing Foden was hurt in an otherwise innocuous looking man-and-ball tackle by Bath's Tom Biggs six minutes into the second half of a closely fought Aviva Premiership match at the Recreation Ground on Friday.
Referee Andrew Small called for the TV match official to take a closer look at the incident but he confirmed there was no foul play.
That was no consolation to Foden, though, who was in severe pain as he was helped off the pitch. He later left the ground wearing a surgical boot.
Mallinder said: "It's an ankle injury but I don't know a lot more about it than that.
"The doctor thinks that it's not broken. We're hopeful it's just a strain."
Stephen Myler made the most of his return to the team with five penalties while Foden's replacement, Tom May, added a late drop goal.
The match also saw the return of England lock Courtney Lawes, his first appearance since February.
Mallinder added: "It's great to see Courtney out there. He missed the end of last season with a shin injury and dislocated his elbow pre-season."
Mallinder acknowledged his team did not have things their own way, saying: "Bath have a good line-out and got stuck into ours. But any win away from home is pleasing.
"In terms of performance we're still not playing as we want. We've got a run of two home games coming up though and after a short turn-around - Sunday to Friday - to get the win is pleasing."
While Saints are now three wins from three in the Aviva Premiership title, it was Bath's first defeat of the season despite taking the lead in the first minute through Nick Abendanon.
Skills coach Brad Davis said: "Our overriding feeling is one of massive disappointment. It was a match of two halves as far as our kicking game was concerned.
"To be brutally honest, Northampton deserved to win the match. But Abendanon was outstanding, brilliant."
Scrum coach Neil Hatley added: "We weren't accurate enough at the set piece and we conceded early penalties."
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