I'm not sure I buy that theory. But any time you lose an important member of the bullpen, whether due to injury or ineffectiveness, other guys get exposed. Or they don't. Madson was almost as bad as Lidge when he tried to replace LIdge in 2009, and also blew two saves in the 2008 World Series, but he eventually became a good closer. That makes it easy to think he had to learn some things or get closing experience but it also might have just been a bad month.
And the Phillies were also able to just plug Giles and Seranthony right in. Now Giles is just like any other closer, a patchwork of excellence and injury and trash, and Seranthony's hurt. Coonrod and Brogdon haven't truly stepped up but Joe is also not a manager well-suited to bullpen adaptation (though Kapler was always more orthodox than expected too).
I might actually flip your theory entirely - it's stills the middle relief that's killing us, as most of those blown saves are neither Hector's nor ninth innings, and those failures only magnify the 9th-inning blown saves, or inability of the offense to come back in the late innings (though they've actually done that well, we just remember the times the don't).
For all the angst, June has actually been the best month of the season on paper, and even more impressive when you consider the injuries. 13-13 in April, 12-16 in May, now 11-10 in June. With one in Cincinnati and two at home against the pesky Marlins we'll probably finish 12-12. 13-11 would be nice and 14-10 would mean the team really is playing its most winning (if not "best) baseball of the year, and simply showed its flaws against the better teams (SD and SF on the road, and a Mets team we still played to a total draw). The 0-2 home series with the Nats was the low points; before the 2-4 West Coast road trip they went 7-2 (one game in Cincinnati and the 6-2 homestand).
So the limbo continues...