Red Sox pitcher Wade Miley confirmed that he would be the fourth or fifth starter for the Red Sox to start the season. 

Miley told the media he would make his first start of the regular season against the Yankees, although he didn't specify which game he would start. That series starts after three games against the Phillies. 

That would indicate that Justin Masterson is the Red Sox's third starter, although manager John Farrell hasn't confirmed it.