The brilliant nine-year-old was due to line up in the Bibby Financial Services Ireland Punchestown Gold Cup on Wednesday, but will play no part in the big spring Festival.
Don Cossack’s problem was discovered on Sunday morning once the Gigginstown House Stud-owned chaser returned from a workout on the gallops.
Eddie O’Leary, racing manager for Gigginstown, said Gordon Elliott’s inmate will not return to action unless he is “100 per cent”.
O’Leary said: “He will be treated with the aim to bring him back.
“If all goes well, that will be the plan, but if he isn’t 100 per cent (fit), we won’t be doing it.
“Unfortunately, he won’t now be getting his planned summer holiday as he’ll be recuperating.
“It’s only a small leg (injury), but with tendon injuries it may as well be a big one.
“He’s won us a Gold Cup, and it could have happened before that, so we’ve at least that to be thankful for.
“The same thing happened to War Of Attrition (2006 Gold Cup winner) - we never seem to get them back for a second Gold Cup.
“It’s a terrible blow for Gordon, who is a tremendous trainer and has done an unbelievable job with him - he’s had a fantastic season.”
In what was being billed as the race of the week at Punchestown, Don Cossack was due to renew rivalry with Aintree winner Cue Card, who crashed out when bang in contention for the Gold Cup.