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The professional wrestling website f4wonline.com reported on Saturday that a knee injury had sidelined Fitch, and a person close to the situation has since confirmed the news with ESPN.com.
After a unanimous decision loss to 170-pound champion Georges St. Pierre in August 2008 at UFC 87, Fitch worked himself back into title contention by reeling off five wins in a row.
But a draw to B.J. Penn and a first-round knockout loss to Johny Hendricks in his two most recent outings have removed the 34-year-old Fitch from title consideration.
A win over Simpson, a middleweight who will make his welterweight debut at the July 11 event, would have gone a long way toward putting Fitch (23-4-1, 1 no-contest) back in the title hunt.
It is not yet revealed who will step in to face Simpson (11-3) at UFC on Fuel 4, which will take place in San Jose, Calif.
Middleweight contender Mark Munoz is slated to meet fast-rising Chris Weidman in the main event.