UFC officials confirmed the change to the June 15, 2013, fight card via Twitter on Thursday (May 2, 2013) evening.
Pierson (13-6) has been around the UFC since late 2010, compiling a 3-2 record with the organization over the course of five bouts. While his wins come over respectable competition in Matt Riddle, Jake Hecht and Lance Benoist; Pierson has failed to get over the hump by losing to elite 170-pound contenders Jake Ellenberger and Dong Hyun Kim.
Prior to joining the UFC ranks, Pierson had finished his opponent in all of his professional victories. However, since then all his wins have coming via decision.
Robertson (12-2) is riding high after picking up his first-career UFC victory last February with a submission of Brock Jardine at UFC 157. Prior to the victory over Jardine, Robertson had gone 0-2 in the world's premier mixed martial arts (MMA) organization and had his head on the chopping block.
The 29-year-old managed to keep his job in dramatic fashion by pulling off a modified kneebar on Jardine to earn the "Submission of the Night" bonus at UFC 157.
He will now look to continue that momentum when he travels north of the border to compete for the very first time.
The 170-pound is expected to take place on the preliminary portion of the UFC 161 card, which airs on FX and Facebook. The main card airs live on pay-per-view (PPV).
UFC 161: "Barao vs. Wineland" goes down on June 15, 2013, at the MTS Centre in Winnipeg, Manitoba, Canada. The main event will see interim bantamweight champion Renan Barao put his title on the line for the second time against former World Extreme Cagefighting (WEC) 135-pound titleholder Eddie Wineland.
For more news and notes on UFC 161, be sure to check out our complete event archive here.