Brennan was designated for assignment by the Red Sox on Sunday.
Brennan was called up to Boston on Thursday and fared well in his lone appearance, allowing three hits in three scoreless innings Saturday against the Blue Jays. However, since he would likely be unavailable to pitch the next few days, the Red Sox decided to add another arm to their bullpen in Ryan Weber, ultimately leaving Brennan as the odd man out on the 26-man and 40-man rosters.