Phillies manager Rob Thomson said Nelson suffered a left hamstring injury during his start Saturday with Triple-A Lehigh Valley, Todd Zolecki of MLB.com reports.
Nelson previously opened the season on the Phillies' injured list due to a left hamstring strain before he was activated May 4 and optioned to the minors. After making three consecutive outings for Double-A Reading, Nelson moved into the Triple-A rotation Saturday and gave up one run over four innings, but the hamstring issue resulted in his day ending early. The extent of Nelson's injury isn't yet known, but it wouldn't be surprising if he was forced to miss at least one turn through Lehigh Valley's rotation.