The Nationals' first addition of September wasn't a prospect, but because they're able to boost their roster by as many as 15 players, they were in position to make a quick roster move on Friday before the end of the minor league season.
They activated left-hander Doug Slaten from the 15-day disabled list, bringing the reliever back after he'd missed 77 games with left elbow neuritis. He had allowed two runs in 11 innings over nine rehab appearances, after struggling at the beginning of the season. Slaten had allowed 15 of 30 inherited runners to score; he'd permitted just four of 26 to score last season, in an impressive first year as the Nationals' lefty relief specialist.
He'll get a chance to prove that the start of his season was a fluke during September, and it's potentially a big month for the 31-year-old, who should hit arbitration after this season.