WEST PALM BEACH, Fla. -- Outside of injuries and players’ health, there’s one burning question on everyone’s mind around Astros camp: Who will back up Martín Maldonado come Opening Day?

And now that Houston’s Spring Training squad is complete, with reliever Rafael Montero reporting to camp Friday following visa issues, that competition is thrust to the forefront.

There are two main backstops in the battle: Yainer Diaz and Korey Lee. The pair, ranked as the Astros’ Nos. 3 and 7 prospects per MLB Pipeline, made their Major League debuts last year. Lee got the call in July and played in 12 games (seven starts) while Diaz got the nod in September, getting into six games (two starts, both as designated hitter) with only two appearances behind the plate.

“I'm excited for what both of them bring to the table,” said righty Hunter Brown, who played with both at Triple-A Sugar Land last year. “Obviously they're both hard-working individuals and they have a lot of talent, so it'll be interesting to see how that plays out.”

• Brown poised to take advantage of opening in rotation

In 104 games at Triple-A last year, Lee slashed .238/.307/.483 with 25 homers and 76 RBIs, striking out 127 times in 446 plate appearances. What stands out, though, is Lee’s defense. The rookie boasts an impressive arm (scouting grade was 70 last year), which enabled him to catch 17 runners stealing at Sugar Land last season (66 games at catcher).

• Lee seeking more 'opportunities to shine'

On the other hand, Diaz’s offensive numbers last year -- a .294/.343/.587 slash line over 48 games at Triple-A, plus an .898 OPS with 25 homers and 96 RBIs between Sugar Land and Double-A Corpus Christi -- are eye-catching. And Diaz has made strides defensively since last season -- he’s looked solid during camp and will get plenty of reps as Spring Training continues.

“I've improved in all my defense compared to last year,” Diaz said via interpreter Jenloy Herrera. “My framing, my throwing, the transfers, and then especially just to be able to call pitches, be able to call the game to attack batters.”

The Astros, as they’ve shown time and time again, value defense over offense when it comes to their catchers. Lee had slightly more experience handling the big league staff last year, but Diaz is able to play first base -- which could come in handy.

Maybe manager Dusty Baker said it best on Friday: “The No. 1 job of catchers is to catch and call a game, and hopefully throw some guys out, stop their running game.

“Hitting is a bonus, big-time. … But the better they call a game and the more they're in charge -- whoever's catching is my on-the-field general, he's second in command to me. So, No. 1, you got to catch.”

How to take control behind the plate and with the pitching staff is something both Lee and Diaz have learned from Maldonado. Witnessing the veteran catcher at work, seeing how he carries himself and builds relationships with the Major League pitchers has been invaluable for the pair.

“I’ll try to help them both,” Maldonado said. “I feel like they both are the future of this team and my job is to take those two guys under my wing and try to teach them and [give them] whatever they need to win, and communicate with them equally. I feel we are a unit, whoever makes the team.”

For Lee, learning “how to be a professional catcher every day” from Maldonado and having some MLB experience enabled the No. 7 prospect to enter camp with a different mindset.

“Rather than like, coming up your first year and you're feeling like you're tiptoeing around [the clubhouse] and you don't really know what's going on. Now, I have it all figured out,” Lee said. “I'm more comfortable with the guys around the locker room and I [know] what I need to do every single day, and I have a good routine about that.”

Keeping in mind Baker’s “catchers have to catch” mentality, it would seem that Lee is the most likely to make the big league roster out of Spring Training, but there’s still plenty of spring left. Plus, if veteran left fielder Michael Brantley’s right shoulder isn’t healed enough for him to be on the Opening Day roster (though the club thinks it will be), both Lee and Diaz could make the club out of camp.

With Grapefruit League play beginning Saturday as the Astros face the Mets at 12:05 p.m. CT (live on MLB.TV), coaches and fans alike will get a taste of both rookie catchers.