The reigning National League Cy Young Award wimmer is 12-4 with a 2.68 ERA and 121 strikeouts but has struggled recently, losing three of his last four starts.
"I'm not disappointed," Arrieta said of not being named the starter. "I'm realistic. My last three starts were poor."
The Cubs are trying to keep Arrieta fresh for an anticipated October run after he admittedly tired late last season.