Pivetta will start Monday's game against the Mariners, Julian McWilliams of The Boston Globe reports.
Pivetta was an option to pitch behind opener Brennan Bernardino during Sunday's game against the Giants, but he didn't enter the game and will instead serve as the starter for Monday's series opener in Seattle. Over his last three outings, the right-hander has struck out 19 in 12 scoreless innings.