Diaz, wearing No. 76, batted ninth in manager John Farrell's lineup.
"It was an amazing feeling," said Diaz. "You can imagine something you work for your whole life and I actually got the opportunity to do it, and it's just very exciting."
In Boston's 6-2 loss, Diaz wound up going 0-for-3 and was robbed of his first hit and likely RBI on a terrific diving play by Jays center fielder Colby Rasmus in the fifth inning.
He was also involved in one of the key plays of the game in the seventh. The Red Sox asked Diaz to drop down a safety squeeze, which he did. But Jarrod Saltalamacchia was thrown out at the plate.
"I wanted to get the guy in. I felt like it should have been more to the right to make sure the run scored," Diaz said.
Diaz didn't have to wait long before the ball found him in the field, making the first play of the game on a grounder to third by Jose Reyes.
Diaz, who is kindly listed at 5-foot-9 and 165 pounds (Dustin Pedroia probably has an inch or two on him), has stayed in pro ball through his work ethic and defensive versatility.
His familiarity with Farrell and Red Sox coaches Torey Lovullo and Brian Butterfield were beneficial in this case. Diaz was drafted by the Blue Jays in 2006 and stayed with that organization until signing with Boston as a Minor League free agent on Dec. 7, 2012. Diaz was with the Red Sox during Spring Training.
"He's had a full career around the infield, and he's a guy that's well aware of our system," said Farrell. "He's had experience with Butter, myself and Torey in Toronto last year as well as this Spring Training here with us. In fact, this is a guy who has worked probably seven to eight years to get to the big leagues, and this will be a special day for him."
Diaz had been relaxing at his Rhode Island home with his wife and three daughters when the call came that the Red Sox needed him at Fenway Park. It then became a whirlwind Saturday afternoon for Diaz.
"I tried to call my parents," said Diaz. "They are out of the country. But I don't even remember, honestly, it was such a whirlwind. I called my family and friends as soon as I could on the way over here."
Diaz is a .228 career hitter in the Minors with 14 homers and 245 RBIs.
To make room for Diaz on the roster, the Red Sox designated long man Clayton Mortensen for assignment.