The Nats are expected to place Leon on the 15-day disabled list on Tuesday and recall catcher Carlos Maldonado from Triple-A Syracuse. They will need to make a corresponding roster to move to make room for Maldonado on the 40-man roster.
The Nationals had a 4-1 lead when the injury occurred. The Padres had runners on second and third with one out, when Orlando Hudson singled to center field. Yonder Alonso scored easily, but Chase Headley barreled into Leon and scored the third run of the game for San Diego.
"Coming in, you intend to slide until there's nowhere to go," Headley said. "He really left me nowhere to go. You hope that he's going to be OK. It's a bummer to have that happen in your debut."
Leon, 23, was clearly in pain and had to be helped off the field by assistant athletic trainer Mike McGowan and bench coach Randy Knorr.
"It was a clean play. He was waiting on the throw, and the runner beat the throw," manager Davey Johnson said. "[Leon] is such an outstanding young man. It was his first big league game, and [he was] all pumped up. He is hurt in his first game, that's tough."
After the game, Leo respectfully declined to talk to the media and was seen on a rolling wheel, which prevented him from putting pressure on his ankle.
It marks the second injury by a Nationals catcher in the last three days. On Saturday, Wilson Ramos tore his ACL while going after a passed ball against the Reds. He is expected to be out for the season.
"Boy, I don't know what's happening," Johnson said.
Jesus Flores replaced Leon and went 1-for-3 in the game. Flores knows what it's like to have an ankle injury. He had a similar injury in 2008 that forced him to miss the final 22 games of the season.
"What can I say? It's really frustrating watching my teammates going down -- one getting run over by another guy, and Wilson hurting his knee," Flores said. "It's like, is something going on? Hopefully, it will stop happening."
As for Maldonado, he has hit .211 with two homers and six RBIs in 21 games for Syracuse.
Bill Ladson is a reporter for MLB.com and writes an MLBlog, All Nats All the time. He also could be found on Twitter @WashingNats. This story was not subject to the approval of Major League Baseball or its clubs.