"It's unfortunate Gronkowski the horse will not be able to race in the upcoming Kentucky Derby due to an illness", Gronkowski wrote on Twitter.
Gronkowski, who qualified for the race through a first-time slot held open for a European-based colt, had gained a considerable USA following for his name - and the subsequent decision of New England Patriots tight end Rob Gronkowski to buy a share of the horse from his ownership group, Phoenix Thoroughbred Limited.
The ownership group tweeted that the horse was treated with antibiotics and "is doing well".
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Gronkowski the horse was sired by champion Lonhro, Australia's Racehorse of the Year in 2003-'04, and was bred in Kentucky as the first foal from the mare Four Sugars, a daughter of twice champion American racehorse Lookin At Lucky.
The three-year-old son of Lonhro has incurred a slight infection and will not be able to make the worldwide journey to Louisville.
Before we have any idea which horse will arrive in Baltimore as the Kentucky Derby champion, we know one who's likely to line up as a fresh contender to knock him off.
"We are beyond disappointed that Gronkowski will miss the Kentucky Derby", said Tom Ludt, who directs Phoenix's global operations. "I'm glad that he's recovering quickly, and I look forward to meeting him soon and taking some selfies!"