The bizarre situation came at the par-3 17th hole at Harbour Town Golf Links in Hilton Head Island, South Carolina, in the US tour’s second event after ending a three-month coronavirus shutdown last week.
Watson blasted his tee shot into a greenside bunker, but when he arrived to check out the ball, he found a tiny crab had crawled under it.
McIlroy, a four-time major winner from Northern Ireland, and Watson were in a featured pairing at the event, meaning tour television cameras were watching the entire situation as Watson pondered his options.
They included blasting the ball out with the tiny crustacean lurking in the sand beneath it and facing animal cruelty complaints.
“I don’t want the world to get mad at me, know what I mean?” Watson told McIlroy as the European star stuck the top of putter near the ball to try and distract the crab.
Watson called for a rules official but as he approached, the little crab flipped out from beneath the ball, landing a few inches away and moving off and eventually crawling away.
“There was a camera and people get mad if I hit an animal,” Watson told the official to explain the problem that had resolved itself.
“We don’t need crab issues, know what I’m saying?”
Watson, 41, eventually blasted out to 17 feet but missed his par putt and took his last of four bogeys in a two-over par 73 round to stand on three-under 210 for 54 holes, sharing 70th.
The stress was unusual for the Heritage, which is typically a relaxing week for players following the Masters, which Watson won in 2012 and 2014.
McIlroy two-putted for par after the crab distraction, then birdied 18 to shoot 66 and stand on 203.