The team said Rojas was ruled out of action due to a nagging back issue.
"I couldn’t have been happier to get the call to come drive again with the team in the No. 01 car in Indianapolis," Karam said. "It’s an unfortunate situation for Memo, and I hope he recovers quickly, but I’m happy to step up and contribute any way I can."
Rojas anticipates he will be back in the car at Road America.
"I spoke to the team after our last race at Mosport and let them know that I didn’t feel that I was 100 percent," he said. "I discussed the situation with our managing director Mike Hull and we agreed that it would be best for me to rest my back this weekend. I know Sage will do a great job filling in and I’m already looking forward to the race at Road America after a few weekends off."
Karam, the Ganassi development driver, will make his third start in the No. 01 Prototype (Daytona, Watkins Glen) and fourth overall with Ganassi this season. This will mark the first time just he’ll be in the car alongside Scott Pruett.
Additionally, Karam will be making his second start at the Indianapolis Motor Speedway this season. He made his IndyCar debut in the Indianapolis 500, qualifying 31st and finishing ninth.