Mortensen was optioned to Triple-A Pawtucket on July 4, so he will have to return to the Minors after Saturday because optioned players can't be recalled to the Majors for 10 days -- except in the case of injury or, like Saturday, a doubleheader.
The Red Sox could've made other additions besides Mortensen but felt he was the best fit to help them for the day.
"Everything," manager Bobby Valentine said of what went into the decision. "There's a million ... There are conversations into the morning that, we're just guessing at what we might need, and what we might need the most. So we decided to go with Mortensen, who's a pretty durable pitcher, can pitch against both sides of the plate, and see if we can't have enough pitching to get through a doubleheader."