Shishkin has been ruled out of next week's Tingle Creek by trainer Nicky Henderson after failing to 'spark' in training.
The unbeaten chaser was due to make his seasonal reappearance in the Sandown Grade One, but Henderson does not feel he is quite yet at concert pitch.
As a result he will now head for the Desert Orchid Chase at Kempton over Christmas instead.
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"I've just spoken to (owner) Joe Donnelly and we've sadly and reluctantly decided that Shishkin is not going to run in the Tingle Creek," said Henderson.
"There's nothing wrong with him, but I just haven't got there. He worked this morning and he just isn't sparky at the moment. I think I just got behind the ball a little bit.
"We did his wind in the autumn which wasn't a problem, but it just put us behind the ball and Nico (de Boinville, jockey) and I both feel he's not quite on his A-game.
"It really is a thin line. We've scoped him and everything and there's nothing wrong, but you can't afford to go into this (at) anything bar 100 per cent."
Henderson: Epatante facing tougher task
It is set to be a big weekend for Henderson as former Champion Hurdle winner Epatante returns to Newcastle to defend her Fighting Fifth crown, live on Sky Sports Racing.
The star mare is looking to bounce back from three successive defeats and has undergone surgery on a back issue over the summer.
"We're all set to go," said Henderson. "We've been very happy as her work has been good and she had a nice away day at Newbury. As with last year, I think she's in good form.
"She was very impressive beating Sceau Royal [last year] but he was very good at Wincanton the other day so I don't think it's going to be that straightforward this time.
"The first part is the Fighting Fifth then hopefully that takes us to the Christmas Hurdle and then you have to start worrying about Honeysuckle again."
'Jonbon making headlines for wrong reasons'
Henderson is also set to unleash exciting bumper winner Jonbon, a full brother to multiple Grade One winner Douvan, for a highly-anticipated hurdles debut at Newbury on Friday.
"He's unaware of the pressure and I don't quite know how it's got to this stage," Henderson said. This is a horse that has won a point-to-point and a bumper.
"At the moment he is getting headlines for all the wrong reasons.
"We took him to Newbury the other day and he's quite a busy horse so I was in two minds whether to just let him walk around the paddock and go home.
"He got a bit warm but was pretty good. It probably did him good.
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"When he ran in his bumper we were still under Covid restrictions so there were actually more people on the press day than the day he won the bumper.
"His race [on Friday] was scheduled to be the first race but it's actually now be moved to the third, only because this hype has brought him onto the television, which only puts more pressure on me, him and everyone else."