The English-based three-year-old -- who earned a spot in arguably the most famous horse race in the world by leading the European points series -- contracted a fever last weekend and is being treated with antibiotics, his ownership group, Phoenix Thoroughbreds, said in a tweet.
"We very much regret that Gronkowski will miss the Kentucky Derby after spiking a fever over the weekend and being treated with antibiotics," it wrote. "He can't make the long journey to Louisville but is doing well."
Bred in Kentucky, the horse was named after New England Patriots Pro Bowl tight-end Rob Gronkowski.
When the football player found that out, he bought a stake in the colt last week and was due to meet his namesake at Churchill Downs in the buildup to the race, which takes place May 5.
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This is the first year a horse from Europe was due to take part in the Kentucky Derby, although Gronkowski the equine wasn't one of the favorites. Last week Gronkowski was an 80-1 longshot in Las Vegas for the 144th running of the race, well behind favorite Justify.