Rossi will share the No. 7 Acura ARX-05 DPi with full-time Penske drivers Helio Castroneves and Ricky Taylor in the Rolex 24 at Daytona and 12 Hours of Sebring.
Rahal had been the co-driver for those events last season but said on social media Friday that he's too big to properly fit in the car. Rahal is listed at 6-foot-2 and posted a post-workout photo last month acknowledging he's the tallest active driver in IndyCar and was "determined not to be the fattest too."
Rossi is a former Indianapolis 500 winner who has slowly been expanding his schedule beyond IndyCar. The American won karting championships and the Formula BMW World title as a teenager, pursued Formula One in Europe, and migrated to IndyCar in 2016.