It's a half-odd switch for the Red Sox. On the one hand, there's no surprise in seeing Aceves go. A disaster outing on Tuesday cemented the fact that the team could not trust him to pitch, especially with John Lackey expected to return to his rotation spot over the weekend.
What's not so clear is why Ryan Lavarnway is coming up. Having given Aceves the boot, it wouldn't have been surprising to see another bullpen arm come up temporarily, especially since the Sox would presumably not mind designating Aceves for assignment to free up his roster spot. Alternatively, they could have added an outfielder (though, again, the 40-man space would have to be cleared) in case Shane Victorino misses a couple of games with the back injury that's been nagging him for a few days.
It doesn't really seem like the Red Sox would call up Lavarnway if they weren't intending to find some playing time for him, however. While the 25-year-old catcher struggled tremendously in his time in the major leagues last year, he's not a lost cause, and is actually off to a very good start in Triple-A, hitting .310/.423/.452. Missing a few days wouldn't kill him, but it's just an odd move all around.
Still, in the end what matters is that Alfredo Aceves is, at long last, gone.