CLEARWATER, Fla. --- Righthander Sean O'Sullivan will start Tuesday's Grapefruit League game against the Toronto Blue Jays in Dunedin, essentially making him the favorite to land the Phillies' temporary fifth starter's job.

The fifth starter the Phillies open the season with will more or less be a stop-gap until Chad Billingsley is ready to join the rotation. Billingsley, coming off a pair of elbow surgeries, is expected to be ready in mid-April or early May.

The Phillies don't need a No. 5 starter until April 12.

O'Sullivan, 27, is an option because he has spent the last couple weeks starting games in minor league camp. Non-roster invitee Kevin Slowey, 30, has major league starting experience but has yet to be stretched out as a starter. He will pitch Monday against his hometown Pittsburgh Pirates and could see an extra inning or two behind pitching prospect Zach Eflin, who is starting the game.

O'Sullivan pitched 12 2/3 major league innings for the Phillies last year. He recorded a 6.39 ERA. Twenty-five starts at triple-A Lehigh Valley yielded a 4.31 ERA.

The way the rest of the week shakes out, opening day starter Cole Hamels will make his final spring start Wednesday in Clearwater against the Atlanta Braves.

Jerome Williams or David Buchanan will likely start Thursday's spring finale. Whomever doesn't would then likely start Saturday night's exhibition against the Pirates at Citizens Bank Park.