Throughout the season, Danny Espinosa’s continued struggles at the plate begged for an explanation. Why was he slumping again? What happened to his reworked swing from spring training? How much was his fractured wrist from April affecting him? Was it actually the torn rotator cuff in his left shoulder that was discovered last offseason?

Health certainly played a part in Espinosa’s fall from grace this season. He experienced swelling and pain in his right wrist after being hit by a Paul Maholm pitch on April 14, and played through the discomfort for more than a month before receiving more specialized treatment and rest. Then, after some pushback, as General Manager Mike Rizzo put it, Espinosa finally went on the disabled list on June 4 after hitting .158 over 44 games. He received MRIs on his shoulder and wrist, but the Nationals treated only his wrist.

He rehabbed at Class AAA Syracuse and stayed there for the rest of the season, not even meriting a September call-up. All along, Espinosa maintained that his shoulder wasn’t bugging him, and the Nationals concurred, focusing mainly on the injured wrist. When Espinosa first arrived at Syracuse, the wrist was still sore. By early August he said it was much improved and he was focused on improving his hitting. He said then that his shoulder was fine. Later that month, Espinosa reiterated that his torn rotator cuff would not require surgery and that it hadn’t limited him at the plate, instead maybe preventing him from building overall upper-body strength in the offseason.

But how could that be? How could torn tendons — we’re unsure on the amount and degree — in his left shoulder not have affected his hitting?

First of all, let’s establish this: Espinosa didn’t just slump in 2013. He hit 21 home runs in 2011 but posted a .236 average and struck out 166 times. In 2012, he hit 17 home runs but with a .247 average and an NL-leading 189 strikeouts. Power is a coveted ability, especially at second base, and 38 home runs in your first two full seasons is good. Batting average is overrated but it’s clear Espinosa struggled with consistency. He has admitted that he developed bad habits in his time in the majors.

The first half of his first full season in 2011, Espinosa was an NL Rookie of Year candidate. Since then, he has slumped. His triple slash line since: .228/.295/.365 with 25 home runs and 82 RBI. That’s good for a .660 OPS over nearly two seasons. Over those 1,105 plate appearances, Espinosa has drawn only 76 walks and struck out 318 times. The struggles with his swing didn’t start after he hurt his shoulder or wrist, but perhaps they just worsened.

So let’s get back to Espinosa’s shoulder. Asked about the second baseman’s shoulder, Rizzo said this at the GM and owners meetings in Orlando: “Espinosa is checked out. His shoulder is healthy and he’s ready to commence his baseball activities to get ready for spring training.”

But still, I wanted to help explain the medicine behind rotator cuff injuries. Perhaps it would reveal some insight into the continued confidence of Espinosa and the Nationals in the rehabbed shoulder. One doctor explained to me during the season that tears of the rotator cuff don’t necessarily mean the shoulder is completely compromised. Pedro Martinez, for example, and a pitcher nonetheless, spent time on the disabled list in 2001 with a right rotator cuff injury but didn’t need surgery until 2007.

I called up Peter Millett, an orthopedic surgeon who specializes in shoulders at the Steadman Clinic in Vail, Colo. and has worked with football, baseball, basketball players and Olympic athletes. Millett knows nothing about Espinosa’s case but was asked generally to explain rotator cuff injuries and their effects. According to Millett, the severity of the injury ultimately depends on the size of the tear.

The rotator cuff is the series of four tendons which are attached to the shoulder, keep the ball in the socket and help control the rotation of the arm. A tear can be anything from small fraying on one small area to a complete break of the tendons off the bone. It’s unclear how severe Espinosa’s tear was, but he did consult with Lewis Yocum last winter, the surgeon whose second opinion revealed the tear after using contrast dye in an MRI.

The decision not to perform surgery was made collectively by Espinosa, his agent, Rizzo and the Nationals medical staff. Those tendons rarely heal on their own. The belief was that if the muscles around the rotator cuff were strengthened, Espinosa would be fine to play this season. Recovery from surgery would have taken four months. Surgery is obviously far more invasive and would require missing a significant amount of time.

“He’s come through all the stress tests that we give him, the strength tests and he feels good,” Rizzo said in spring training. “He’s a ball player. He knows what he can take and what would hurt his performance and he feels good about entering the season.”