Left-hander Ricky Romero will make his season debut for the Blue Jays on Friday after the club placed Johnson on the 15-day disabled list on Thursday.
The right-hander missed his last start due to inflammation in the triceps of his pitching arm. The move is retroactive to April 29.
"Yesterday we thought he was going to be OK," Blue Jays manager John Gibbons said of Johnson. "He went out, played some catch, throw his bullpen, but then he started complaining 'You know what, it really hasn't gone away.'
"He was still a little bit concerned about it."
Johnson is 0-1 with a 6.86 earned-run average in four starts since being acquired in a blockbuster off-season trade with the Miami Marlins.
"He'd been feeling pretty good, but he hadn't thrown in a while, but then he played catch on the side (on Wednesday) and it felt pretty good but he still hadn't aired it out," said Gibbons. "He was airing it out a little before stepping on the mound and he heard alarm bells."
Romero has 125 career starts with Toronto, earning a career 51-43 record and a 4.09 ERA. However, the 2011 all-star struggled in spring training and started the season in the minor leagues to work on his mechanics.
He will start the opener of a three-game weekend series with the Mariners at Rogers Centre against Seattle ace Felix Hernandez.
Romero had one start with the single-A Dunedin Blue Jays in 2013, allowing one run on six hits in seven innings.
"(Blue Jays roving pitching instructor Dane Johnson) felt he was ready to go," said Gibbons. "Ricky feels good, he's anxious to do it."
Hours after the Blue Jays made the announcement Romero excitedly tweeted about the start.
"Go Time!!! #ForeverBlessed," said Romero from his verified Twitter account with a link to a photo that read: "Dear Past, thank you for your lessons. Dear future, I'm ready. Dear God, thank you for another chance."
Toronto also recalled right-hander Brad Lincoln from triple-A Buffalo and assigned righty Justin Germano.
Lincoln has made two appearances for the major league club this season, allowing two runs in two innings.
Gibbons said Romero will not be limited to a pitch count in Friday's start.
"He's strong," said Gibbons. "We won't get carried away, but he's ready to go."
The Blue Jays manager was unsure of how long Romero would stay up with the big club, saying the team is just focused on Friday's start.
Toronto dropped a 3-1 decision to the Boston Red Sox on Thursday night to fall to a disappointing 10-19 on the season.Suggest a correction