FORT MYERS, Fla. -- From the shoulder to the ankle to the oblique to the foot, Rusney Castillo's body betrayed him in a variety of areas last year and prevented anyone from getting much of a read on the player the Red Sox invested $72.5 million in back in August 2014.

So perhaps it makes perfect sense that Castillo's goal for the coming season is a simple yet important one.

"The goal is to stay healthy this year, because last year the injuries kind of hampered me a little bit," Castillo said through an interpreter. "But moving forward, I'm just going to work on staying healthy and making sure we have a good season."

Over these next few weeks, Castillo has the chance to prove to manager John Farrell that he should be the team's primary left fielder.

Competition will come from Travis Shaw, Chris Young and Brock Holt for at-bats in that spot in the lineup.

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"He's still not an established Major League player," Red Sox president of baseball operations Dave Dombrowski said of Castillo. "He has to go out and show he can earn the job. And the way you go out and do it is to play day in, and day out. He has to go out and do it. Do we think he can do it? Yes. But he still has to perform and show us that he can."

Castillo participated in his first full-squad workout on Saturday after missing the first three days with the flu. But he'll take an unavoidable illness over the various nags that troubled him with both Triple-A Pawtucket and the Red Sox last year.

Determined to never have a year like that again, Castillo rededicated himself this offseason. Maybe Castillo thought he worked hard enough in previous winters, but the lesson he learned was that he needed to do more.

"I prepared physically in the offseason to make sure I was ready for this season," Castillo said. "So this year, I worked out really hard. I didn't focus much on the physical stuff but more so on the conditioning aspect to last the season. I've done a regimen in the offseason to make sure that I stay healthy."

What kind of player can Castillo be if he stays on the field?

"If I stay healthy, I can run well, I can swing the bat well and I can play well in the field, so I'm happy to do all those things," Castillo said.

There was a month-long stretch -- from July 27 through Aug. 24 -- when Castillo showed all of those things, hitting .375 with four homers, 19 RBIs and a 1.024 OPS.

After that point, however, Castillo's offense disappeared. He hit .185 with no homers and four RBIs over his final 119 at-bats of the season. So what happened?

"The thing that surprised me was the injuries I suffered last year, it forced me to stop my conditioning and that's why by the end of the year, I was more tired and wasn't really in baseball shape," Castillo said. "So basically I was unstable, and being away from the field two to three weeks at a time because of injuries really threw me off my rhythm."

Assuming he can stay in rhythm this time around, how much does Castillo plan on playing?

"I really want to play a lot," Castillo said.