Triple-A Rochester placed Fernandez on its 7-day injured list May 14 due to an unspecified injury.
The 31-year-old reliever made three appearances for the Nationals back in late April but has otherwise spent the entire season at Rochester, with whom he's turned in a 3.27 ERA, 1.00 WHIP and 10:5 K:BB in 11 innings across 13 appearances. Since he hasn't appeared in a game in over a month, Fernandez could require a rehab assignment in the lower minors before returning to the Triple-A bullpen.