Baylor was set to catch the ceremonial first pitch from Vladimir Guerrero, who signed a one-day contract prior to the game to retire after a 16-year career.
Guerrero then threw the first pitch to Baylor, the only other league MVP in Angels history (Baylor won the award in 1979). The throw from Guerrero was low and a bit outside, Baylor reached for the ball in the dirt and then appeared to injure his right leg while making the catch and standing up.
Guerrero tried to help him up and two Angels staff members had to help him off the field.
"It got me sick when I was warming up and hearing what happened in the dugout," said Mike Trout, who would go on to hit a two-run homer and finish 2-for-4.
Baylor, 64, was taken to UCI Medical Center and diagnosed with a fractured right femur. The team announced he would have surgery on Tuesday.
"Obviously it's a freak (injury)," Angels manager Mike Scioscia said. "We'll see how it turns out."
Baylor, who played 19 seasons in the big leagues, was the Colorado Rockies' first manager and earned the 1995 NL Manager of the Year and his 500th career victory with the Cubs in 2000. Baylor left the Arizona Diamondbacks in the offseason to return to Anaheim, where he played six seasons with the California Angels.
He joined the Angels staff in October.
Information from the Associated Press was used in this report.