The Red Sox are sending reliever Pat Light to Pawtucket, recalling infielder Marco Hernandez to replace him on the roster.
So there's a few things at work here.
First, Hernandez is a great fit for the roster right now. The Red Sox could use a lefty bench bat with a solid infield glove, and Hernandez has also been getting some outfield work in. In the event the Red Sox have Chris Young up late in the game against a righty reliever and don't want to or can't pinch-hit the would-have-been starter for him for whatever reason, John Farrell could do worse than Hernandez.
Second, Pat Light is just the last man in the pen right now. Nothing surprising about that since he's a fresh call-up and (understandably) struggled in his first appearance. Realistically he'd have needed to blow the doors off the place to stay up in the first place, and beset by debut jitters, he started wild against the Braves, surrendering a pair of runs. He'll get another chance at some point, but he was the obvious one to go down at the moment.
Third...John Farrell might just be starting to feel comfortable with his bullpen. At least that's the implication with the Red Sox finally sending away their extra arm to return the bench to full strength. And one has to imagine that's in no small part because Heath Hembree has stepped up in such a big way. Robbie Ross Jr. and Matt Barnes are also turning in decent innings, but it's Hembree with his nine scoreless innings that has really made the difference. If Farrell needed his alternative to the Uehara/Tazawa combination in the pre-Kimbrel innings, Hembree has certainly earned that role, striking out eleven while allowing just six baserunners in the process.
And oh, yeah, Carson Smith is on the way back too. It was a little sketchy for the bullpen there to start the season, but things finally seem to be settling in as April comes to a close.