The 25-year-old had to be withdrawn 12 minutes into his side's 18-14 victory at Franklin's Gardens - Bath's first triumph on Northampton soil for 16 years.
Speaking after the match, Todd Blackadder admitted that the initial signs were not positive but would wait for tests before making further comment.
"It sounds like a little bit of a medial, so we'll just wait and see," he told BBC Sport. "It's about the strength in depth of your whole squad and we showed that."
Faletau joined the Aviva Premiership outfit from Newport Gwent Dragons earlier this summer.