Stepping in for Pierson will be newcomer Quinn Mulhern (18-2) who makes the leap from the Strikeforce cage to the Octagon at the Montreal show.
Promotion officials confirmed the bout by changing the lineup for the pay-per-view event at UFC.com on Tuesday.
Mulhern Tweeted (@Qmulhern) the news as well, indicating a possible injury to Pierson as the reason why he pulled out of the fight.
UFC 158: St-Pierre vs. Diaz is expected to take place on March 16, 2013 at the Bell Centre in Montreal, Quebec, Canada. The event is headlined by a welterweight title match between UFC champion Georges St-Pierre and former Strikeforce champion Nick Diaz.
Mulhern is 3-1 over his last four matches, all of which have come under the Strikeforce banner. Mulhern was a former KOTC welterweight champ with eleven submissions, three (T)KO and four decision victories to his name. His most recent win came in May of last year at the Barnett vs. Cormier event which saw Mulhern win a split decision over Yuri Villefort.
With the influx of Strikeforce fighters into the UFC, Story may be facing dismissal should he lose again. ‘The Horror’ is 1-3 over his last four matches with a first round submission loss to Demain Maia this past October at UFC 153. The win in that stretch was over replacement newcomer Brock Jardine at UFC on FX 4 last June. The other defeats came at the hands of Martin Kampmann at UFC 139 and to Charlie Brenneman at UFC on Versus 4.