From the Washington Post's Nationals blog:
Two weeks ago, the Nationals returned Terrell Young, their Rule 5 draft pick, to the Cincinnati Reds. But once Young reported, he complained of shoulder tendinitis -- with symptoms similar to those that held him out of spring training, while with Washington. Because of that injury, Major League Baseball has voided Young's return, meaning that the reliever is back on Washington's 40-man roster. And back on the 15-day disabled list. On Thursday, orthopedist James Andrews will perform exploratory surgery on Young's right shoulder.
I'll let acting GM Mike Rizzo elaborate...
"The situation is, we sent him back to Cincinnati as part of the Rule 5 return policy. And when we sent him back there, he was throwing successfully and pain-free. When the Reds got him, he discovered he had a little tendinitis in his shoulder. So they contacted us, and we asked them to bring him back to us and have our doctors see him again... We did find a continuation of the tendinitis, and Dr. [James] Andrews recommended we do an exploratory surgery to see what, if anything, is going on."
Asked about whether Young was healthy at the time of his return to Cinci, Rizzo said, "Well, he went on his throwing program, so we ramped him up to game speed. He pitched in one game for us and said he felt great, everything was good, so we deemed him healthy and we sent him back to the Reds."