'It doesn't look good': Leeds Rhinos' Jack Walker suffers new injury blow during loan at Hull FC
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Walker, 23, opened the scoring in Hull’s 60-6 home defeat by St Helens on Sunday, but hobbled off in the first half.
Hull coach Brett Hodgson reported: “He felt a crack in his ankle.
“We don't know the extent of that yet, but it doesn't look good.”
Walker, a Grand Final winner in 2017, missed much of the 2020 campaign and all of last season due to a foot injury which threatened to end his career.
He battled back to play in Rhinos’ opening nine games of this term before picking up a hamstring injury during a home draw with Huddersfield Giants in April.
After being given the all-clear, he joined Hull last month on a loan deal until the end of this season.
Walker has scored two tries in six appearances for Hull and Hodgson added: “He has been good for us in this short period we've had him.”
Meanwhile, Rhinos are awaiting the result of a scan on the hamstring injury suffered by centre Harry Newman in the second half of last Friday’s 28-20 win at Hull KR.
The England World Cup hopeful was distraught in the changing room after another blow in a disrupted campaign which has seen him limited to just seven senior appearances so far.
Newman, who battled back from a broken leg sustained in September, 2020, suffered damage to a hamstring in a pre-season game at the end of January and the injury reoccurred during his first Super League run out of the season, at Wakefield Trinity in early March.
He returned away to Huddersfield three months later, but picked up a three-match suspension after just two games of his comeback.
Cameron Smith, who started at stand-off against Hull KR and forward Bodene Thompson both went to hospital on Friday evening and were discharged after treatment.
Thompson suffered a rib injury.
Smith had stitches in a thumb wound, but did not undergo surgery.