Onomah suffered a hamstring injury during Wednesday's 1-0 win at Middlesbrough on Boxing Day and could be out of action until February, according to the new Owls assistant manager Steve Agnew.
Speaking at Friday morning's pre-Hull City press conference, Agnew told the official Sheffield Wednesday Facebook page: "He (Onomah) is back with Tottenham at the minute. He's had treatment. He's had scans. But I think, from what I've just heard, he'll be about maybe another month away."
Onomah's Owls spell has been stop-start throughout, although he had looked a player rejuvenated following Jos Luhukay's departure as manager last month.
And the 21-year-old, who has created three goals in 11 games with his temporary employers, will have been looking forward to pushing on under Steve Bruce, the new Wednesday manager who signed him for Aston Villa.
Agnew also confirmed that Onomah's midfield colleagues Barry Bannan and Sam Hutchinson will be fit for Saturday's trip to Hull, with Lucas Joao and Fernando Forestieri expected back in training next week.
How big a miss will the Tottenham player be for Sheffield Wednesday?
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