Casey Janssen has had issues with his shoulder before. (Reinhold Matay/USA TODAY Sports)

VIERA, Fla. — Washington Nationals reliever Casey Janssen is expected to undergo an MRI exam on his sore right shoulder by Monday, Manager Matt Williams said. Janssen has had trouble getting his shoulder loose recently and felt some discomfort during his last outing on March 23.

Janssen, 33, signed a one-year, $3.5-million deal with the Nationals in the offseason, expecting to take over as the eighth inning replacement for Tyler Clippard. Janssen has made six spring appearances, allowing three runs on six hits while striking out five and walking two. Janssen was listed on the Nationals’ pitching schedule to face the Mets on Saturday but was later removed.

Janssen has had shoulder issues in the past. He had minor right shoulder surgery — to repair soreness in his AC joint — in 2012 and had some shoulder soreness to start last season. He still managed a first-half ERA of 1.23 in 23 games before struggling with a 6.46 ERA in the second half. Janssen missed the first month-plus of last season with a lat strain, the muscle that wraps around the side of the body and stretches up near the shoulder.

“Generally it’s in his lat,” Williams said of Janssen’s shoulder issues. “It’s not something that’s normal soreness for him.”

Should Janssen’s MRI exam reveal an injury, the Nationals have ways of filling the void, perhaps solving the Nationals’ bullpen competition in spring. Aaron Barrett and Blake Treinen could now both likely make the opening day roster,  joining Drew Storen, Craig Stammen, Jerry Blevins, Matt Thornton and Tanner Roark. Barrett has had a good spring, Thornton has experience as a set-up man and Stammen has experience, but team officials have been impressed with Treinen’s electrifying spring. Rafael Martin, a right-handed non-roster invitee, and Xavier Cedeno, a left-hander out of minor league options, are also still in camp.

“We built a lot of depth there though,” General Manager Mike Rizzo said earlier in the day, before news of the Janssen MRI was revealed. “We’ve got a lot of guys that are very capable of pitching very late in games. We feel comfortable and good with where we’re at late innings and in the ballgame.”

Even if Janssen only needs rest, it comes as time is running out before the season begins in a week.

“Relievers are different because they don’t need as much time,” Williams said. “But I want to make sure he’s feeling good about it and do everything he wants to.”

The Nationals have suffered through a brutal spring of injuries, from Denard Span (core muscle) to Anthony Rendon (knee) to Stephen Strasburg (ankle) to Drew Storen (left hamate bone surgery), in addition to rehabbing Jayson Werth (shoulder) and Nate McLouth (shoulder).

>>> In Sunday’s 11-7 win over the Marlins, Doug Fister gave up six runs on eight hits and one walk over four innings. Uncharacteristically, Fister gave up three home runs, bringing his total to five this spring, and he made two defensive mistakes. Fister made a delayed throw to second after fielding a grounder with a runner on, and also misplayed a grounder. “We probably won’t see two plays like that the rest of the year,” Williams said.

The wind at Space Coast Stadium was strong in the early afternoon, seemingly turning every flyball into a home run. The shot that Fister gave up to Giancarlo Stanton felt like a pop-up to the outfield but ended up just over the left field seats. Regardless, Fister has a 7.02 ERA (13 earned runs in 16 2/3 innings) this spring.

“Basically command, mostly on the sinker, making sure that it’s down and on the corners,” he said. “It was up a little bit too much [Sunday]. Gotta keep it down but in and out it wasn’t too bad. I’m throwing my breaking ball a little bit better. A little bit better with the cutter also. Seeing some good lights at the end of the tunnel but obviously there’s going to be little things that I need to finish and make sure I take care of.”

Fister will make one more start, the exhibition game in Washington on April 4, before the season begins. He will then have a week before he makes his first start of the regular season.

“At this point in spring it’s kind of a good thing to get an extra day,” he said. “Kind of recoup or throw a couple extra bullpens, kind of fine tune things down there versus in a game. Even if that means seeing if we can get a couple hitters on a back field to just stand in on a bullpen or whatever it may be. There’s other options not throwing in a game can do for us.”

>>> Barrett gave up a run on two hits in one inning of work. Thornton (one inning), Rich Hill (one inning) and Stammen (1 1/3 innings) each had scoreless outings.

>>> Michael A. Taylor and Bryce Harper each had good performances at the plate. Hitting leadoff, Taylor went 3 for 3 with a double and homer. Harper went 2 for 3 with a double and homer. He has also drawn 12 walks this season. Wilson Ramos also went 3 for 3 with a home run. He was replaced by Jose Lobaton, who crushed a three-run home run in the eighth inning.

“It’s all about good contact,” Williams said. “… It’s good to have that as we get closer. The fact that we’re staying on the baseball middle of the diamond is good. I liked all of that. It certainly helps our confidence to get hits. The results are great but it’s about their approach. I thought they were good today.”

>>> Including the two errors committed on Sunday, the Nationals have made 25 errors in 26 spring games. Oddly enough, two occurred the same day the team took extra situational defense drills. Williams prides himself on good defense and preparation so he hasn’t been too pleased with the team’s performance so far.

“There’s major room for improvement,” Williams said. “It’s one of the things that sticks in my craw a little bit because we work so hard on it. But nonetheless, we’ll keep improving and keep going and keep working. We work on it every single day. We just keep at it. It’s all we can do.”