Star chaser Sprinter Sacre has been ruled out of the Tingle Creek Chase at Sandown on 6 December, leaving uncertainty over his return.
Trainer Nicky Henderson said he could be fit in time for Kempton over Christmas, the race in which his heart complaint first surfaced.
The eight-year-old could also race in the Clarence House Chase in January.
Henderson said: "He's in full exercise and work, but he isn't quite ready and we are not going to push him."
"He wouldn't be in training if we didn't think we were going to be racing again."
Sprinter Sacre was rated one of the most brilliant chasers, winning races at the 2013 Festivals at Cheltenham (Queen Mother Champion Chase), Aintree (Melling Chase) and Punchestown (Champion Chase).
However he has not raced since being dramatically pulled up in the Desert Orchid Chase, Kempton last Christmas with an irregular heartbeat.
He is joint favourite with 2014 Cheltenham Champion Sire De Grugy - also currently on the injury list - for this season's Cheltenham championship.