Simon spent three years with the Reds, the first two as a reliever. He moved into the rotation in 2014 and went 15-10. The Reds traded him to Detroit for infielder Eugenio Suarez, and he went 13-12 with a 5.05 ERA for the Tigers in 31 starts.
Injuries have left the Reds with several open spots in the rotation. Homer Bailey isn't expected back until May as he recovers from Tommy John surgery. John Lamb had back surgery and won't be ready for the start of the season. Michael Lorenzen was diagnosed with a sprained ligament in his right elbow and is sidelined indefinitely. Lorenzen was examined on Thursday and won't throw for at least two more weeks.
Plus, Jon Moscot was scratched from his scheduled start on Wednesday with soreness in the right side of his rib cage. He'll get a couple of days off.
"Our history with Alfredo made this a much easier decision," manager Bryan Price said. "We know what we're getting. He is extremely resilient. His durability is a huge bonus.
"He gives us depth in the rotation. There are just too many question marks now with the readiness of some of our young guys that we don't want to force to the big leagues too soon, and there is the injury component."
Simon has been throwing in the Dominican Republic. He sent the Reds a video of his 50-pitch session. Price thinks he can get ready to start quickly.
"In the three years he was with us, he was the most durable arm on our staff," Price said.
The Reds traded starters Johnny Cueto and Mike Leake last July, leaving them with an all-rookie rotation for the second half of the season. They had hoped to add some veterans to the rotation and the bullpen in the offseason, but hadn't been able to do it.
"We've had our eye on the pitching market all spring and will continue to look at it if there is someone that can help us," general manager Dick Williams said. "Before we got to camp we did want to supplement the ability of our rotation to throw innings and not put pressure on any pitcher to have to ask to do more than he is ready to do."