Mastroianni, who was promoted from Triple-A Buffalo on Saturday morning, hit a two-run homer for Toronto in its 4-3 loss to the White Sox. The shot to deep left-center field was even more impressive considering it came off Chicago's No. 1 starter Chris Sale.
The Blue Jays added Mastroianni to the roster to provide some more depth vs. left-handed pitching. That's something the club has been sorely missing with Jose Bautista and Brett Lawrie currently unavailable because of injuries.
"To be honest with you, I didn't even know they were throwing a lefty today until about 11:45 p.m. last night," said Mastroianni, who went 1-for-3. "I got the call to say I was coming up and I checked and saw Sale on the probables. I was like, 'Oh boy, here we go.'"
Mastroianni's time in Toronto could be relatively short-lived. Bautista is expected to make his return to the lineup on Tuesday, but Mastroianni could save his spot by beating out Anthony Gose to become the club's fourth outfielder.
The 28-year-old Mastroianni has spent parts of four seasons in the Major Leagues. He is a career .218 hitter but has the ability to play all three outfield positions and has been relatively effective vs. lefties in a small sample size.
"I want to try to help," Mastroianni said. "This team is in first place, it's all about winning here, try to do my best to help us win. We just fell a little short there at the end."
Toronto optioned left-hander Rob Rasmussen to Buffalo in order to make room on the 25-man roster for Mastroianni. Rasmussen has appeared in seven games for the Blue Jays this season and has allowed four runs with seven strikeouts and four walks over six innings.
The latest move means the Blue Jays will temporarily go with a six-man bullpen. Another move could be on the horizon, but if the bullpen remains rested then it's likely that the club will carry the six relievers until Bautista returns.