Adrian Wojnarowski of The Vertical broke the news via Yahoo Sports on Friday, reporting Bosh has withdrawn from the All-Star Game "with an unspecified injury" and Horford is now en route to Toronto.
According to Jeff Zillgitt of USA Today, Bosh suffered a strained calf.
Bosh earned a spot as a reserve on the Eastern Conference All-Star team, a triumphant moment for the 31-year-old whose final game last year came in an All-Star jersey before he was diagnosed with a blood clot on one of his lungs.
It was also going to be a nice bit of poetry for Bosh, whose career started in Toronto when the Raptors made him the fourth overall pick in the 2003 NBA draft. He spent seven seasons with the franchise before heading to Miami.
Even though Toronto fans' relationship with Bosh since he left has been chilly, he told Shandel Richardson of the Sun-Sentinel before the league announced the All-Star rosters that he would like an opportunity for closure: "Just for me personally, it would mean a lot to me just to continue to play at a high level but to kind of come back to where it all started and maybe give me some All-Star closure."
While Bosh's return to Toronto for the All-Star Game would have been a fun storyline to follow, Horford is a worthy replacement.
The Hawks star is averaging 15.3 points and 6.9 rebounds per game and ranks sixth among Eastern Conference centers in ESPN.com's estimated wins added (8.0). This will mark his fourth appearance in an All-Star Game since 2010.
The East roster is loaded, with LeBron James, Paul George, Carmelo Anthony, Dwyane Wade and Kyle Lowry starting, so head coach Tyronn Lue should feel confident about his team's chances even without a versatile big man like Bosh to come off the bench.