The 31-year-old was forced to limp off during the Canaries' win over Wigan last weekend and the club will access the striker's fitness over the next couple of days
The striker has scored four Premier League goals and started every game this season, helping his side reach seventh place under new manager Chris Hughton.
"Yes, we're worried. You're worried about any injury," the ex-Newcastle boss told BBC Radio Norfolk.
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"It's a concern - and a bad time - but it's not right to speak about it in bad terms when we don't really know.
"Hopefully he'll be OK. if not, the competition within the side continues."
In Holt's absence it may well be left up to in-form players Anthony Pilkington and Wes Hoolahan provide the goals if Norwich are to continue their recent good run of form.
Pilkington, who scored the first and then assisted Hoolahan for the second goal against Wigan, spoke fondly of his team-mate who recently signed a new contract.
"He's a great player. He's probably the best player I've played with," he added.
"You see his foot work and how good he is on the ball. It's great that he's here for the next three years - it's great for him and great for the club."