Mike Rizzo, right, watches a bullpen session. (Toni L. Sandys/The Washington Post)

For the first time this offseason, the Washington Nationals made a move likely to affect their major league bullpen. On Tuesday, the team traded a rookie-ball reliever to the Tampa Bay Rays for left-hander Enny Romero. Romero is not a lock to make the Nationals’ Opening Day bullpen but seems the likeliest to secure a spot of all the candidates the organization brought in this winter. The 26-year-old’s fastball sat at 96 mph last season — in the top 20 among relievers who threw at least 40 innings — and he struck out 9.85 batters per nine innings. Of current Nationals relievers, only Shawn Kelley and Sammy Solis struck out more in 2016.

The Rays deemed Romero dispensable after trading for outfielder Logan Morrison, who did not have a spot on the 40-man roster until Tampa Bay made the move Tuesday. They will receive 21-year-old right-hander Jeffrey Rosa from the Nationals’ Gulf Coast League affiliate, according to a person familiar with the situation. Rosa pitched to a 4.91 ERA in 11 Gulf Coast League appearances last season.

Romero’s command will likely determine whether he makes the Nationals’ Opening Day bullpen. His 1.79 strikeout-to-walk ratio last season was worse than any regular Nationals reliever. For reference, similarly unpredictable lefty Oliver Perez’s strikeout-to-walk mark was 2.30. But Romero has the swing-and-miss stuff the Nationals hunt for in late-inning options. He does not project as a matchup lefty but rather as a full-inning type, and he fared far better against right-handed hitters than lefties last season (.217 batting average against as opposed to .288).

If he were to make the Opening Day bullpen, he would join Perez and Sammy Solis to form a trio of left-handers, something Dusty Baker treasured when it materialized at times last season, praising the versatility it allowed him in the later innings. As it stands, the Nationals seem to have four sure things in the bullpen: Solis, Perez, Kelley and Blake Treinen. Koda Glover seems likely to make it, too. Including Romero, the Nationals would have one more spot to fill — likely that of a long-man — and would be surrounded by six pitchers with strong fastballs and strikeout capabilities.

But the Nationals still do not have a clear-cut closer, a void they have yet to fill despite establishing it as a priority earlier this offseason, and appear to be betting on the growth of less experienced pitchers.

That Nationals General Manager Mike Rizzo and his staff opted to pursue a trade to bolster the bullpen, rather than spend in what began as a deep free agent relief market, fits with what has been a rather stingy offseason for the Nationals. Though many within baseball see the Nationals as a player or two away from World Series contention, the Nationals have spent $4.9 million on two bench players — key players, to be sure, but not game-changers. Still, Romero is as close to a big league acquisition as the Nationals have made for their bullpen, and will compete with an intriguing group of non-roster invitees for a spot in the opening day relief corps.