The addition of Patrick Corbin gives the Nationals three prized arms at the top of their rotation. (Matt York/Associated Press)

The Washington Nationals had a rail-thin rotation for all of four days, from the time they traded depth starter Jefry Rodriguez to the Cleveland Indians to when they added baseball’s top free agent arm Tuesday.

Left-handed starter Patrick Corbin is expected to join the Nationals, pending a physical, and his contract reportedly includes an average annual value of $23 million over six years. Corbin joins Max Scherzer and Stephen Strasburg to give the Nationals three front-line starters atop their rotation.

The 29-year-old is coming off a strong season that included an 11-7 record, a career-low 3.15 ERA and 246 strikeouts in 200 innings for the Arizona Diamondbacks. He also was an all-star for the second time and finished fifth in National League Cy Young voting. And now that the Nationals have crossed off the biggest objective on their offseason list, they can shift their attention to filling out their staff for 2019 (along with exploring the market for a left-handed first baseman, potentially adding another piece to the bullpen and seeing whether there is still a chance of bringing Bryce Harper back on a long-term deal).

There are many ways the Nationals can address their rotation from here. Tanner Roark is the logical fourth starter, while the fifth spot is not as clear-cut. The only certainty, given how aggressive they have been this offseason, is that they will give themselves a number of options. Joe Ross should have the inside track to be the fifth starter, but the Nationals also could consider Erick Fedde and look for low-cost, high-reward veterans to compete for innings in spring training.

“Starting pitching is king,” General Manager Mike Rizzo said Saturday, a few days after the team hosted Corbin and a few days before it outbid the Philadelphia Phillies and New York Yankees to add the lefty to their roster. “Our philosophy is pitching, defense, athleticism. That’s how we’ve won. When we put our guy on the mound, [and he], each day, gives a chance to win, you’ve created yourself a chance to have a really good ballclub and play deep into October. That’s our philosophy.”

Before delving into the back end of the rotation, it’s worth noting just how strong the top now is. Scherzer is one of the league’s best starters and led the NL in wins (18), innings (220.2) and strikeouts (a career-high 300) as he finished with a 2.53 ERA in 2018. He also has won three Cy Youngs and finished second to Jacob deGrom in 2018. Strasburg was hampered by injuries last season and saw his fastball velocity dip into the low 90s when he returned from the disabled list in August. But when he’s healthy — and if his velocity ramps back up, as the Nationals expect — he can be the dominant pitcher who finished with a 2.52 ERA in 2017. And there are clear reasons Corbin, a proven ace, was so coveted this offseason.

That all means the Nationals are not pressed (or expected) to scour the free agent market for another costly starter, such as Charlie Morton or J.A. Happ. Roark is coming off an up-and-down season that included a dismal start and a post-all-star-break stretch in which he went 5-1 with a 1.61 ERA. He is at his best when keeping his sinker low in the zone and, having twice won 15 games in a season, provides upside as a fourth starter. Ross is more of a question mark after making just three starts after a 14-month recovery from Tommy John surgery. Ross gave up nine earned runs in the 16 innings he pitched in September, but he also flashed reminders of why he was a relied-upon starter in 2015 and 2016.

“He’s got to come back to spring training and work his way back,” Manager Dave Martinez said Saturday when asked whether he expected Ross to be in the 2019 rotation. “I liked what I saw, I did, but he’s got to build stamina. What he showed me, after three or four innings, third time around, his [velocity] went down significantly. We’ve got to get him consistent and get him through that third time through the order.”

If that doesn’t happen, and Ross is still shaky in spring training, the natural next option is Fedde. The 25-year-old was one of the Nationals' top prospects heading into last season, made 11 starts, took a turn on the 60-day disabled list and is now trying to increase his durability by increasing his weight. But the Nationals will need more than just Fedde and Ross, regardless of how the pair looks heading into the season, because this team has shown it needs at least seven or eight starters to get through a season.

That is why the Nationals signed Henderson Alvarez to a minor league contract in November, betting low on a past that features a no-hitter at 23 years old and an all-star appearance at 24. Alvarez is 28 now, his once-promising career has been dashed by repeated injuries, and he spent last season in the Mexican League. But he is the kind of expendable veteran arm — think Jeremy Hellickson and Tommy Milone last season — that the Nationals stockpile for depth. Hellickson is a free agent and could return on a cheap one-year deal for the same purpose, and there are many available pitchers of similar caliber.

The Nationals also have shown interest in 24-year-old prospect Wil Crowe, who finished this past season with Class AA Harrisburg and made his first appearance at the team’s annual WinterFest this past weekend. That usually signals that a player could be close to getting a shot at the majors. The Nationals have not shied away from jumping players from Class AA to the big leagues, and they will explore any and all ways to make their staff as strong as possible.

The rotation is surely much better off than it was at the start of the week. But, as always, there is still some work left to do.

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