Brian Barry-Murphy says midfielder has ankle ligament damage.
Colwill limped off after only 20 minutes and was seen leaving Sixfields on crutches.
Bluebirds boss Brian Barry-Murphy said the problem "looks pretty serious on first glance".
"He damaged his ankle ligaments," Barry-Murphy added. "I think the nature of the way the injury occurred suggests it won't be a short-term injury."
Post match manager comments:
