Faugheen has been ruled out of his reappearance in the BHP Insurance Irish Champion Hurdle at Leopardstown on Sunday with a pulled muscle.
Willie Mullins' 2015 Champion Hurdle winner has been off the track since winning the Leopardstown showpiece 12 months ago.
His season has been interrupted by a series of niggling problems, and it appears another one has struck.
With 2016 Champion Hurdle winner and stablemate Annie Power also on the sidelines, a frustrating season for Mullins shows no signs of letting up.
"I hope it's not serious," Mullins told Press Association Sport.
"It's a muscle on his hind quarters that he's pulled and he'll hopefully be all right in a few days.
"It's frustrating, as he'd done everything right at home and was ready to run.
"While it's only slight, I just dare not risk him."
Mullins indicated Faugheen would now head straight to Cheltenham.
He added: "The previous problem was in his foot and this is a completely different area. He'd worked well Tuesday and on Thursday - all was fine. He'd nothing to do only go to the races, but this morning he pulled out a little lame behind.
"It's unfortunate, but I couldn't risk running him even if he pulled out sound in the morning. It wouldn't be fair on him.
"I doubt he will get a run in before Cheltenham. I'm hoping this will only take a day or so. It's just disappointing that it's happened at this time and no reason as he was sound after working on Thursday.
"You don't expect these things, but they do happen unfortunately. They are fragile animals."
Mullins will still be represented in the Leopardstown feature by the Graham Wylie-owned Nichols Canyon.
The Authorized gelding has been a fantastic servant to connections, winning at Grade One level on no less than seven occasions.
He inflicted a shock defeat on Faugheen in last season's Morgiana Hurdle and successfully defended his crown in that Punchestown contest in November.
However, he proved no match for Petit Mouchoir in the Ryanair Hurdle at Leopardstown's Christmas meeting and now has something to prove as a result.
Wylie said: "It's a fact-finding mission because he was disappointing at Leopardstown.
"Take nothing away from Petit Mouchoir, who ran a great race in a fast time, but we don't think Nichols Canyon ran his race.
"We'll see how he goes on Sunday and it will tell us whether he's a Champion Hurdle contender or we need to step up in trip."
Nichols Canyon is entered in both the Champion Hurdle and the Stayers' Hurdle at Cheltenham.
The Henry de Bromhead-trained Petit Mouchoir, a former Mullins inmate, could hardly have been more impressive over the festive period.
De Bromhead said: "He seems in good form. Obviously it's a big test for him, but we'll go and take our chance and see how we get on."