Joe Ross turned in another quality start Thursday and, for now, will remain in the rotation for the Nats. (Brittany Greeson/The Washington Post)

Nationals Manager Matt Williams announced after Thursday’s game that rookie Joe Ross will remain in the starting rotation when Stephen Strasburg returns Saturday, and Doug Fister will move to the bullpen. Criticized at times this season for relentless loyalty to struggling veterans like Ian Desmond, Ryan Zimmerman, and others, the Nationals chose meritocracy as they replaced their ERA leader from 2014 with a relatively unproven rookie who has never stopped proving himself in seven big-league starts.

“[Fister] wants to pitch. He’s willing to do whatever he can to help us pitch,” Williams said. Fister sat in the bullpen Thursday afternoon. Since his rookie season with the Mariners in 2009, Fister has made three relief appearances — and 167 starts. The 6-8 right-hander known for his work ethic and professionalism has struggled this year, landing on the disabled list and battling mechanics in his contract year. That Fister will be a free agent means this move comes at a critical juncture in his career. That he has battled mechanics in a delivery that features many moving parts means that relieving may be tough. Without regular work or a predictable schedule, syncing things back up may prove challenging.

“It’s a difficult task when you’ve been a starter for so long,” Williams said. “But the opportunities will be long. That’s kind of where we see it. It’s never easy, but he’s willing to go out there and do what he can to help us win a ballgame.”

Ross said he did not know he was staying in the rotation when reporters asked him about it after the game.

“I feel good. Honored,” Ross said. “It’s tough with the five great starters that we have and Strasburg coming back, so for me to stay, obviously, I’m gladly going to take the role and continue to try to help the team.

Ross is 22 years old and began this season at Class AA Harrisburg. He has not pitched more than 122 1/3 innings in a professional season. After Thursday, he is at 121 innings. The Nationals have said they will keep Ross on an innings limit, though neither he nor they have specified what that limit might be.

Williams said Wednesday if Ross is in the majors, he will not be limited on a game-by-game basis because the Nationals cannot afford it. While in the minor leagues, he might be removed from the game after five innings to satisfy his innings limit, he will be handled as a regular starter in the majors — because in the midst of a pennant race, the Nationals need as many well-pitched innings as they can get.

“Obviously, I want to go out there and do my best and hopefully have to make them make a decision,” Ross said. “But if I were to, if Stras were to come back and take over his spot, I would have completely understood and been fine with it.”

In seven major league starts, Ross is 3-3 with a 2.80 ERA and 47 strikeouts in 45 innings. He has walked four batters, and his WHIP is below one.

“Joe? I’ve been catching him a lot this year, and right now, this is one of the guys we need in the rotation, definitely,” catcher Wilson Ramos said. “Amazing guy. He’s throwing really good. He helps the team a lot, so that’s what we need right now. We need guys who can help the team.”

When Strasburg returns to start on normal rest Saturday, the Nationals will have an extra reliever, because Ross was called up when Strasburg went down. Fister will join Tanner Roark, Blake Treinen, Felipe Rivero, Matt Thornton, Casey Janssen, Drew Storen and Jonathan Papelbon in the bullpen. The Nationals may choose to option out one of the younger relievers with options — Roark, Rivero or Treinen, for example — and bring them back when rosters expand in September. They may opt to carry a short bench during that time instead, though at this point cutting back on a bench player would likely mean losing that player, as neither Tyler Moore, Dan Uggla, or the rest have options to spare.

Williams did not say what will happen when Ross hits his innings limit. He said the Nationals will decide whether Fister or someone else returns to take that spot if that becomes necessary. For now, the Nationals are choosing the hot hand over the veteran one, as a 22-year-old elbowed his way into one of the deepest rotations in baseball.