KISSIMMEE, Fla. -- Considering the number of jobs that are up for grabs in Astros camp, this year's Grapefruit League campaign shouldn't be lacking intensity or intrigue as 63 players compete to be among the 25 the team carries to Houston in a month.

The Astros, who endured an offseason full of change on and off the field, will begin trying to sort out their long list of uncertainties when they meet the Washington Nationals at Osceola County Stadium at 12:05 p.m. CT on Saturday.

"I know once the games get there, the marathon starts," outfielder Jason Bourgeois said. "I'm excited, and I feel great going into Saturday and am just ready to take advantage of any opportunities."

Among the questions the Astros will need to answer: Who's the starter at third base? How healthy is catcher Jason Castro? Who will fill out the pitching rotation and bullpen? What role will Brett Wallace play? The next month will begin to provide some clarity.

"We're going to evaluate them all," manager Brad Mills said.

Veteran right-hander Livan Hernandez, likely to be in the rotation to start the year, will start Saturday against his former team in the first of 15 scheduled home games in Kissimmee for the Astros. They will play their first road game Sunday at 12:05 p.m. against the Nationals in Viera, Fla., in a game available live on MLB.TV.

"There's no doubt I expect some intensity," Mills said. "These guys are battling for positions and at the same time are trying to get ready for the season."

Perhaps the biggest spring storyline is the health of Castro, who missed all of last season after injuring his right knee in Grapefruit League play and undergoing ACL surgery. He had surgery on his left foot in December, which slowed his progress. Castro will start Saturday and is scheduled to catch three innings on the one-year anniversary of his knee injury.

"We'll be checking with him every day and seeing how he's doing," Mills said. "We're going to try to bring him back slowly. That's something we as a coaching staff are going to have to throw around. How many days do we give him back-to-back before the end of spring? Your catcher has to go back-to-back before the end of spring, but that might be dicey because of his situation."

The Astros added depth at catcher by signing veteran Chris Snyder to join Castro and Humberto Quintero, but only two of those catchers will break camp with the team.

Chris Johnson, last year's Opening Day starter at third, will start Saturday in his effort to win back the job he lost midway through last year to rookie Jimmy Paredes. Getting sent to the Minor Leagues last July didn't well with Johnson, and he feels he has lots to prove.

"The games are definitely important," Johnson said. "I'm trying to win a job. I also have to remember they know what I can do and they're looking for me to step up and grow a little bit. That's what I'm trying to show them. They're going to see how I react when I struggle and see if I've moved on."

Wallace is in much the same boat as Johnson. He started at first base on Opening Day last year, but was sent to Triple-A with Johnson at the end of July. Both Wallace and Johnson will get time at first base and third base this spring. Wallace led the Grapefruit League in RBIs last year with 18 and hit .377.

Things are just as unsettled in the outfield, where J.D. Martinez is likely to start in left field. Jordan Schafer is the favorite to win the center field job, but Brian Bogusevic and Bourgeois are in the mix. Newcomer Jack Cust is among those competing for time in right field.

"We're going to be getting everybody some looks," Mills said. "We have [Matt] Downs, [Joe] Thurston and [Brian] Bixler who can all play the outfield. We've got a number of guys we can run out there and get a look at them. We'll try to get everybody at-bats in the first week or so and piece it together."

The pitching rotation cleared up somewhat with veteran starter Brett Myers being moved to closer. As it stands, Hernandez, Wandy Rodriguez, Bud Norris and J.A. Happ are in the rotation, with youngsters Jordan Lyles, Lucas Harrell and Kyle Weiland and veteran Zach Duke competing for a spot.

Lyles, who will start Monday against the Braves, had a terrific spring as a 20-year-old last year and still couldn't crack the Opening Day rotation. He wound up making his debut in May and went 2-8 with a 5.36 in 15 starts. Another good spring could earn Lyles a job on the Opening Day roster for the first time.

"They say you'll earn a spot if you show them you deserve one," Lyles said. "Hopefully what they're saying is what's going to happen. If I can do what I did last year and just get ready for the season, hopefully that will happen. I felt pretty good so far this spring, and I'm excited about the start of games."