Clippard is currently having the best season of his professional career, going 4-1 with a 0.92 ERA in 24 appearances for Syracuse. After six years as a starter, Clippard said he loves his new role.
"I feel comfortable with what I'm doing," Clippard said. "Last year was a struggle for me, working though a lot of different things.
"Coming into a relief role this year, it has helped me forget about everything mechanically -- not having to worry about throwing a bullpen [session] between starts. I'm just going out there and firing the ball and eliminating those bad thoughts. It just kind of turned into a good thing for me. Everything is kind of falling into place so far."
Clippard was scheduled to be called up Sunday because it was believed that right-hander Jesus Colome would go on the disabled list with a right quad injury. In fact, Clippard was in D.C. on Sunday, but he was told to go back to Syracuse because Colome said he was OK after having a bullpen session.
Clippard, who will be a middle reliever for the Nationals, received another call about his promotion on Tuesday night.
"It's good to know that I was in line for this," Clippard said. "Whether it happened today or a month from now, it's good to be here. That's all I care about."
Wells was 0-2 with a 6.49 ERA in 23 relief appearances with the Nationals this season. Wells last pitched on Tuesday against the Red Sox. He pitched two-thirds of an inning and gave up two runs.
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.