A picture of Smalling was posted on Twitter with a bandage around his head, a brace on his neck and a drip coming out of his arm.
The post suggested he had been hurt while having a surfing lesson while other reports said he had been stung by a jellyfish.
The truth, however, proved more mundane and the 26-year-old, who played for England at Euro 2016, has now fully recovered.
“Chris has had food poisoning during his holiday which resulted in him fainting yesterday. He was taken to the hospital and is now feeling fine," a United spokesman said on Monday.
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(Reporting by Neil Robinson, editing by Ed Osmond)
