This is, of course, the proffered explanation for Hanrahan's recent struggles. In his last three games, Hanrahan has allowed six earned runs while recording just five outs, walking four batters and surrendering three home runs. And a hamstring injury is something that could throw a pitcher as far out of whack as Hanrahan has been.
The question is: do you believe it? Hanrahan's control problems are not contained to the last few days. He walked better than five batters per nine innings last year, culminating in September--a month which saw him walk ten batters and allow five runs in nine innings of work.
At the very least, this gives Hanrahan a bit of an excuse. Something that can cool off pressure from fans calling for his head after one blown save. If, however, this is not the whole story, and there's more to Hanrahan's recent struggles than a simple sore hamstring, then the Sox are going to have to get to the bottom of the underlying mechanical issues before they can feel confident in their closer.
The good news is that, in the meantime, there's plenty of depth in the bullpen that the Sox can turn to to take on the ninth inning in close games.