Lidge signed a $1 million, one-year contract as a free agent in the offseason to add veteran bullpen depth behind closer Drew Storen and All-Star Tyler Clippard, but the 35-year-old right-hander couldn't find the form that made him a two-time All-Star with Houston and Philadelphia. He went 0-1 with two saves and a 9.64 ERA in 11 appearances with the Nationals, and opponents were batting .308 against him.
Lidge allowed three runs in Washington's 7-2 loss to the New York Yankees on Friday and was the losing pitcher against New York on Saturday, giving up two runs in the 14th inning of a 5-3 defeat.
General manager Mike Rizzo said the marathon game forced the move, with the bullpen in need of a fresh arm for Sunday's series finale.
"Brad wasn't performing very well," Rizzo said, "and Mattheus was ready to come off the rehab assignment. We thought this was the right time to make the move."
When Storen went down with an elbow injury during spring training, Lidge began sharing the closing duties with Henry Rodriguez, but a sports hernia sent Lidge to the disabled list in late April. He was reinstated on June 7.
"I think he was healthy," Rizzo said. "He said he was healthy. He threw like he was healthy. He was in no pain, no after-effects after he was done rehabbing."
Lidge debuted with the Astros in 2002. He has 225 career saves, and helped the Phillies win the 2008 World Series by going the entire regular season and postseason without blowing a save.
After Saturday's game, Lidge said he felt he was making good pitches, but the batters were finding the holes.
"Just keep throwing good pitches, quality pitches, and at some point those balls will get to people and we'll make outs," Lidge said. "But until then, you've just got to battle and keep throwing strikes."
Mattheus missed the last 24 games with a plantar fascia strain in his left foot. The 28-year-old right-hander went 0-1 with a 9.00 ERA in three rehab appearances with Double-A Harrisburg and Single-A Potomac.
In 19 relief appearances with Washington this season, Mattheus is 2-1 with a 2.25 ERA.