Myhill, on a season-long loan spell from West Brom, sustained the problem during an innocuous incident in the warm-up to Saturday's 2-2 draw against Bristol City.
The seriousness of the injury will be assessed over the coming days but Myhill will definitely miss Tuesday's trip to Ipswich and is highly likely to sit out of Saturday's clash at Brighton, with Colin Doyle taking his place between the posts.
And that has forced manager Chris Hughton's hand in recalling England Under-21 stopper Butland as cover from League Two Cheltenham, where he was due to remain until the end of the season.
Speaking on Monday morning, Hughton said: "We'll assess Boaz over the next couple of days and the extent of his injury, with him being out it was an obvious one to bring Jack back.
"It's not broken, in effect I think he's sprained it but we need to see whether it's more than that."