Tigers have confirmed the 29-year-old - who will join Warrington Wolves next season - has an acute bulging disc in his neck as a result of whiplash suffered in the first half of last Saturday’s defeat by Hull KR.
A spinal specialist has advised six weeks' rest before further scans are taken.
Holmes will miss Thursday's must-win final game of the regular season, at home to Warrington and Tigers' play-off campaign, if they qualify.
“I am heartbroken to end my time at Castleford Tigers like this and to not get one last game at the Jungle with the lads," Holmes - who has been in the first team squad for 12 years - said.
"But this is sport, these things happen and it is what it is.
“I’ve loved my time at Cas and I will be at the game on Thursday night cheering us on for the win and I will be able to say my goodbyes after the game.
"Hopefully, the lads secure the victory, push on in the play-offs and get to that Grand Final."
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