Santana gave up five runs in five innings as the Mets lost the final game of their first homestand of the season, 5-2.
Josh Willingham hit a first-inning grand slam that was ruled a home run only after an instant-replay review. The umpires initially missed the ball going over the centerfield fence because it bounced back onto the field. That cost Barajas when the 6-6, 287-pound Dunn crushed him on what proved to be an unnecessary play at the plate.
Barajas was not injured, but there's no way getting an elbow to the side of the head felt good.
What else didn't feel good? The Mets lost two of three to the Nationals after losing two of three to the Marlins. That inauspicious start could be a problem; the Mets start a six-game trip against 2009 playoff teams Colorado and St. Louis tomorrow.
And losing to the Nationals with Santana against Mets castoff Livan Hernandez (seven shutout innings) hurts as much as a shot to the head. Especially with Washington missing No. 3 hitter Ryan Zimmerman (hamstring).
"We were real confident because we had Santana," said Jose Reyes, 1-for-4 with a walk in his second game off the DL. "But they played better than us."
Jerry Manuel said his team did not make the proper adjustments against Hernandez, whom the Mets released last August. "Today's game was not a good game for us," Manuel said. "I thought we appeared unprepared and I have to take responsibility for that."
The first-inning slam didn't help. Willingham's drive to center hit a second fence behind the main outfield fence; the ball hit above and to the right of the orange home-run lines. Second-base umpire Jim Wolf ruled it in play, however, as it bounced back onto the field and was retrieved by Gary Matthews Jr.
Two runs scored immediately. The Mets got the ball to Barajas in plenty of time for a play on Dunn, but he dropped the throw before being run over football-style by Dunn, who once signed a letter of intent to play quarterback at Texas but chose baseball.
Willingham, who was at third, started for home as the ball trickled behind the plate. Santana retrieved it and threw it back to Barajas, who tagged Willingham as he slid in hard for what was called the second out.
Nationals manager Jim Riggleman argued and the umpires went into the tunnel behind the third-base dugout to check the replay. They quickly came out, and crew chief Derryl Cousins made a circular motion with his right hand to indicate the call had been changed to a home run and a 4-0 - not 3-0 - lead.
"There's a lot of things that happened on that play," said Willingham, who also had an RBI double off Santana in the third inning to make it 5-0.
Dunn had another chance to crash into Barajas. In the third inning, Dunn was on third when Cristian Guzman hit a fly ball to right. Jeff Francoeur uncorked a beauty of a throw on the fly and Barajas tagged Dunn, who tried to tiptoe past him rather than slide - or crash.
The Mets' bats were silent until the eighth, when much-booed Mike Jacobs hit a two-run homer off former Yankee Brian Bruney. But that was it.
"It's kind of disappointing in a sense that we weren't able to solve Livan," Manuel said. "Johan is going to have an inning here or there where things happen. But I thought it would have given us a good lift [to] kind of fight our way back. I thought we had every opportunity to, but we were unable to get it done."