Stepping in for Yousef will be British grappler John Maguire, who's fresh off a UFC on Fuel TV 5 defeat at the hands of John Hathaway.
Prior to the loss, Maguire (18-4) had reeled off two straight victories to begin his UFC career over the likes of DaMarques Johnson and Justin Edwards respectively.
Word of the replacement comes by way of Sherdog.
Riddle (6-3) is a veteran of The Ultimate Fighter (TUF) season seven and previously defeated Chris Clements in Calgary via submission but had his victory overturned by the commission after testing positive for marijuana metabolytes. Even his medical marijuana card couldn't save him.
UFC 154 features a welterweight championship unification bout pitting the recently rehabbed Georges St. Pierre looking to reclaim his 170-pound throne from sit-in interim straphanger Carlos Condit. Division contenders Martin Kampmann vs. Johny Hendricks are scheduled for the co-main event.
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