Sierra takes over the spot that was left vacated when Toronto dealt left fielder Travis Snider to the Pirates on Monday night for right-hander Brad Lincoln.
To make room on the 25-man roster, the Blue Jays optioned relievers Joel Carreno and Evan Crawford to Las Vegas. The latest moves leave the Blue Jays with the eight-man bullpen they have been carrying for most of the past two months.
"When they were trading those guys yesterday it crossed my mind that I might be called up to the big leagues knowing that they probably need somebody in the outfield," Sierra said through interpreter Luis Rivera.
"I'm prepared, I'm looking forward. I'm going to get on the field and show everything that I have, show the potential that I have."
Sierra is expected to platoon with fellow rookie Anthony Gose in right field until Jose Bautista returns from a sprained left wrist. When Bautista does get back into the lineup, which isn't expected to happen at least until the end of the week, Sierra likely will find himself back in Triple-A Las Vegas.
The 23-year-old was hitting .289 with 17 home runs and 63 RBIs in 100 games for the 51s this season. General manager Alex Anthopoulos compared him to Rangers outfielder Nelson Cruz and said he was even considered for a callup last season.
"I didn't know I was compared to Cruz," Sierra said. "In the past, I've been compared with Raul Mondesi, I've heard that before. But I'm very happy knowing that they compare me with a guy that has been successful in the big leagues."
Sierra was signed as an undrafted free agent in 2005 by then-general manager J. P. Ricciardi. He is currently ranked as the club's 15th best prospect according to MLB.com.