WEST PALM BEACH, Fla. -- The Nationals' middle-relief depth has been dealt another potential blow with reliever Justin Miller shut down from baseball activities after a recent MRI revealed a muscle strain in his lower back.

Miller did not make his Grapefruit League debut until this past Saturday’s 10-6 win against the Twins because of issues with his back, but he felt improved enough to take the mound. But he labored through his first inning of game action, giving up a pair of runs on a couple of hits, then his back started giving him issues again after the outing. Nationals manager Dave Martinez did not have a timetable for Miller's return to game action, but said the team does want to handle him with caution. His arm is fine, so the Nats want him to rest and strengthen his back for now before he resumes throwing bullpen sessions.

"He’s getting better, but he's still stiff," Martinez said before Monday's game in Jupiter. "We're going to make sure we can just kind of hone in on that area in his back and get him ready, get him back."

Miller was among a group of relievers in camp competing for the final spots in the Nats' bullpen, and he was one of the favorites. A fast start in Triple-A earned him a promotion to Washington in 2018 for the first time in two seasons, and he was one of the Nationals' most important relievers down the stretch. In 51 games, Miller, 31, posted a 3.61 ERA with 60 strikeouts in 52 1/3 innings. Martinez liked Miller's flexibility out of the bullpen, and he was used at times to throw multiple innings, or pitch in high-leverage situations late in games, racking up his first two saves since 2015.

It’s early to project the roster Opening Day, but without Miller, a thin Nationals relief core just got thinner. Koda Glover also has not resumed throwing after he was diagnosed with a right forearm strain last week.

Martinez plans to rely on Sean Doolittle, Trevor Rosenthal and Kyle Barraclough as his top three relievers -- ideally, he wants to line them up for the ninth, eighth and seventh inning, respectively -- with Matt Grace and Wander Suero behind the trio.

That leaves room for at least two open spots, for which Glover and Miller were likely the favorites. Lefties Sammy Solis and Vidal Nuno and righties Austen Williams and Aaron Barrett are also competing in camp for a bullpen spot, and Martinez has mentioned right-hander Henderson Alvarez as both a starter and a reliever who has piqued his interest.

A year ago, the Nationals relied pretty heavily on Doolittle, Ryan Madson and Brandon Kintzler at the start of the season, and the performance of the latter two suffered as their usage increased. Martinez wants to avoid that scenario this year.

"I like where we're at, I really do," Martinez said. "My biggest concern is keeping these guys healthy. You've got Trevor coming off of Tommy John. Barraclough had injuries last year. Doolittle missed [two] months with a toe injury. Keeping all these guys healthy is the key. If we can stay healthy, our bullpen's gonna be really good."

Doolittle returns to camp

Doolittle returned to camp Monday after taking a few days away from the team to tend to a family emergency.

Doolittle had been out since at least Saturday, but Martinez had told him to take as much time away as he needed. The Nats' closer participated in a full workout with the rest of the team on Monday, and Martinez wants to give him a few days to ease back into camp.

Doolittle is not expected to return to Grapefruit League action until later this week. 

Up next

The Nationals are going to play Brian Dozier and Trea Turner together as much as possible this spring to help the double-play combination form a bond. They are each expected to be in the starting lineup behind Max Scherzer on Tuesday afternoon against the Red Sox at 1:05 p.m. ET.