The Scot, 23, suffered the problem on Friday during Hull's 25-12 win at Warrington Wolves in Super League.
Coach Craig Sandercock now admits that Walker is likely to be ruled out of action for the foreseeable future.
"Unfortunately, he's suffered a pretty serious ankle injury," Sandercock is quoted as saying by Sky Sports News.
"We won't know exactly how long he'll be out until we get the scans back but the physio thinks it is pretty significant and that it will be substantial length of time."
Hull return to Super League action on Thursday when they host local rivals Hull FC.