After a rough past month, Stammen delivers three scoreless innings that allowed Bryce Harper to hit the walk-off shot.
The switch hitter admits he has thought about hitting only from the right side, but would need time to make the switch if he did.
Werth has been playing through an ankle he sprained last week in Miami, but says he will be fine.
Steven Souza Jr. was called up to the majors on Monday and Michael A. Taylor was promoted to Syracuse.
The ruling that went against the Nationals didn't cost them a run, but it shook their faith in the challenge system.
Walters came to Washington in a seemingly minor 2011 trade that sent Jason Marquis to Arizona. He was turned into Cabrera.
The circumstances this season and broadly have made the trade deadline a fascinating moment for the Nats.
Harper is 4 for 7 in the series with Miami and has looked more comfortable at the plate.
The Nationals starter pitched more like himself Monday, firing seven solid innings in his second start back from a bicep injury.
With Lobaton behind the plate this season, Fister is 7-0 with a 1.82 ERA in eight starts.