Nationals' Felipe Rivero: Your new favorite hard-throwing lefty...

In an MLB Network Radio interview this week, Washington Nationals' GM Mike Rizzo talked up Felipe Rivero as one of the power arms from the organization that has supplemented the Nats' rebuilt bullpen this season, throwing 95-98 with command and providing an additional left-handed presence.

Rivero, who turned 24 earlier this month, is up to 9 ⅓ scoreless innings during an impressive stretch over the last few weeks after he threw 1 ⅓ scoreless against the Pittsburgh Pirates tonight, retiring the four batters he faced on 16 pitches, 11 of them strikes, with a fastball that sat around 97.8 mph and got up to 98.6, and a slider that sat between 82 and 85 mph.

Nats' skipper Matt Williams said tonight he was impressed with what he's seen from Rivero, who was acquired along with Jose Lobaton and minor league outfielder Drew Vettleson in the February 2014 trade that sent Nate Karns to the Tampa Bay Rays.

The Nationals converted Rivero to a relief role exclusively this season after he started in the Rays' minor league system and during his first season in the Nats' organization.

"It's a little bit different mentality out of the bullpen," Williams told reporters after the Nationals' 9-3 win in PNC Park.

"He's been a starter for the majority of his minor league career, which means that he's got to conserve as he goes deep in a game. He doesn't necessarily have that worry at this point. So tonight it was really good. 97 mph fastball. That's the best fastball we've seen him have and some breaking balls for strikes to get ahead as well.

"He pitched really well. Good matchups too with the lefties coming up and a lefty-dominated bench for tonight, so good matchups for him too."

It's also helpful that the starter-turned-hard-throwing reliever can give the Nationals more than one inning if/when it's necessary.

"He can pitch multiple innings, he's used to that, so that's not an issue for him and if he's got some lefties coming up we can stretch him to two if we want," Williams said.

On the year, the left-hander now has a 1.96 ERA, a 2.19 FIP, three walks (1.47 BB/9) and 19 Ks (9.33 K/9) in 18 ⅓ IP.

[Rizzo points to head].

• We talked about Rivero's outing, the Nationals' offense, Gio Gonzalez's sort-of abrupt exit and more on tonight's edition of Nats Nightly: