Richard Tolkien, 61, was injured as he tried to clear the sail when it became entangled in his vessel's rigging.
Mr Tolkien, of Lymington, Hampshire, was taking part in the Transat bakerly race from Plymouth to New York.
Mr Tolkien, who was rescued by a cargo ship on Friday near the Azores, said there was "a lot of blood".
"I was down below in my cabin and there was a huge bang. I went on deck to find that the attachment of the second sail... second jib sail to deck, had broken."
"In trying to get the sail under control, I was hit once and then more seriously a second time, a glancing blow to the side of the head."
He called the nearest vessel and was approximately 880 nautical miles west, south west of Horta, when he was found by the cargo ship at about 22:00 BST.
Mr Tolkien praised the captain and crew on board the ship, who cleaned him up and put stitches in his head.
"I was very shocked. The outcome could have been very different," he said.
French sailor Francois Gabart, 33, has already won the race, after crossing the Atlantic in eight days, eight hours, 54 minutes and 39 seconds.
Mr Tolkien, an experienced sailor, previously finished sixth in the monohull class in the 1992 Transat race, but in 2000 was forced to retire in the solo round-the-world Vendee Globe race after running into technical problems.
He also competed in the solo Route du Rhum race in 2010, from St Malo to Guadeloupe.
The cargo ship he is aboard is expected to arrive in Philadelphia on Wednesday. His team is trying to organise  recovery of his yacht.
A British sailor has been rescued from his yacht in a transatlantic race after suffering a head injury.