Tidal Bay is a doubtful runner in Saturday's Argento Chase at Cheltenham.
The 12-year-old won the Grade One Lexus Chase at Leopardstown over Christmas but Paul Nicholls says he is "odds against" to run at the weekend.
The champion trainer told Betfair: "After working this morning, he came back slightly lame. Myself and Clifford (Baker, head lad) have looked him over and the problem is hard to diagnose. It could be either an infection brewing in his off-hind foot, and or he has pulled a small muscle, but we don't know at the moment."
Nicholls added: "We will obviously monitor him in the next 24 hours, and then make a decision on his participation.
"It isn't a serious problem - and he may well yet run on Saturday - but I thought it was best to keep everyone informed. At this stage, though, I think it is odds-against him running at Cheltenham at the weekend."
The Argento has already lost its headline act, with Gold Cup favourite Bobs Worth ruled out by Nicky Henderson.
Nicholls has also raised the possibility JCB Triumph Hurdle favourite Far West may not have another run before the Festival. The four-year-old is unbeaten since arriving from France, winning at Chepstow and twice at Cheltenham.
Nicholls had suggested Far West could run this weekend in the Triumph Hurdle Trial, but he will instead saddle either Sametegal or Irish Saint in that contest.
"I was always going to run one of the other two this weekend. There's no plan with Far West, he's completely A1, though. I haven't been searching for another race for him and it wouldn't be crucial that he even had another run before the Triumph," said the Ditcheat handler.
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