Wells was one of several relievers who struggled during Tuesday night's 11-3 loss to the Boston Red Sox. He walked two, allowed two runs and retired only one batter during the six-run eighth inning, raising his ERA to 6.49.
The Nationals' bullpen this season has a 7-24 record with a 5.59 ERA and 17 blown saves.
Clippard, who joined the Nationals for Wednesday night's game against the Red Sox, was a career starter before his move to the bullpen this year at Syracuse. He was 4-1 with a 0.92 ERA for the Triple-A club and didn't allow an earned run in his past 14 appearances.