The 21-year-old suffered a medial ligament sprain in his side's Johnstone's Paint Trophy victory and has returned to parent club Liverpool for tests, while Town boss Mark Cooper has confirmed he will be out until next month.
Cooper told the club's official website: "Kevin Stewart has a bad one from a bad tackle the other night against Newport.
"Our Physio, Paul Godfrey, made an initial diagnosis of a grade two MCL tear but, as per his loan agreement, Liverpool requested he return to them for further assessment and a scan.
"The scan has confirmed our initial diagnosis and he will remain with Liverpool for his rehab which will take at least six weeks."
Meanwhile, Swindon have boosted their attacking options with the signing of former Notts County striker Jeremy Balmy on a one-year deal.