This comes straight from the Pirates themselves, as everyone that covers the team tweeted out something along these lines almost simultaneously:
#Pirates LHP Wandy Rodriguez has been given a PRP injection and will be on a rest, treatment and rehab plan for three weeks, per team.
— Michael Sanserino (@msanserino) July 1, 2013
PRP stands for platelet-rich plasma. To roll it back a bit further, plasma is the liquid part of your blood (that is, the liquid that carries your blood cells all over your body) and platelets are blood cells that are rich in growth factors. Basically, PRP injections are intended to speed healing in situations involving tissue damage, which you might find in, say, a pitcher's ulnar collateral ligament.
Recently, there's been a pretty well-established pattern of pitchers who've had elbow or forearm problems followed by rest, then PRP-injections, then Tommy John surgery. Ben Lindbergh at Baseball Prospectus investigated just that over the weekend with the news of Dylan Bundy's upcoming surgery. The second paragraph, describing the Bundy's road to the surgical table, should sound awfully familiar to Pirate fans:
It’s been almost three months since we became aware of an issue with Bundy’s elbow. The first reported red flag, “mild tightness,” was followed by an MRI that showed no structural damage, a few weeks of rest, a visit to Dr. James Andrews—who prescribed more rest and a platelet-rich plasma (PRP) injection—and several more weeks out of action. Bundy recently resumed a throwing program, but he suffered a setback that sent him back to Dr. Andrews and, ultimately, the operating room.
Of course, there's no actual evidence that indicates that this is where Rodriguez's injury is headed; all we know now is that his MRI was negative for structural damage and there's nothing to contradict that to this point even though he doesn't feel well enough to pitch. Lindbegh's piece linked above is interesting and I'd recommend you read all of it; at this point a lot of people view PRP injections as time-wasting witchcraft that ends up having no positive impact on the health of the paper, but there's some evidence that partial UCL tears have positively responded to the injections.
All of that being said: there is very little good news to find here for the Pirates. In the study Lindbergh's post discusses, it took an average of 12 weeks after the PRP injections to get back into games, though, again, those pitchers had partial ligament tears and that hasn't been confirmed for Rodriguez at this point. That could theoretically point to a quicker recovery for him, especially given the three-week timeline that the Pirates have apparently mapped out. I'm assuming that that three-week timeline doesn't include a minor league rehab stint, though, so best-case scenario for him is probably still something like six weeks from today. Still, six weeks from today takes us well into August and twelve weeks from today takes us into November. Even if Rodriguez responds to the injection well and avoids surgery, I don't think it's a certainty that he'll pitch for the Pirates again in 2013.