JF seems to think its due to "infrequency of use,"
http://www.csnne.com/boston-red-sox/mujica-struggling-through-infrequency-useMujica has appeared in 10 games and had precious few incident-free outings, with an ERA of 10. Even more alarming is the fact that he's walked four in nine innings, after walking just five in 64 2/3 innings a year ago with the St. Louis Cardinals.
"He was in a more clearly defined role last year,'' said Farrell, "so there was more frequency of use. He got on a little bit of a roll last year and carried it throughout nearly 40 saves last year.
"It has been command (that's been an issue this year) within the strike zone, not just base on balls. It was evident tonight, with the first pitch to Loney - he trying to go down and away and he pulls it across the plate for a double. And we've seen it a few times, where there's been some mislocation. He's starting to gain some arm strength now that we're (almost) through the first month, and increased velocity.
"But still, to me it's more infrequency of use to this point.