MESA, Ariz. -- There is only so much that manager David Ross and the Cubs' evaluators can glean from watching daily workouts in the early portion of Spring Training. Once the games begin, that is when the roster competitions really get underway.

For Saturday's Cactus League opener against the A's, the Cubs will hand the ball to starter Alec Mills, who is vying for either the fifth rotation spot, or a role in the Opening Day bullpen. Mills, who is out of Minor League options, understands that the coming weeks are critical for this next chapter of his career.

"I love it here," Mills said. "So, obviously, I think the best-case scenario is for me to be up here and helping these guys do whatever they can to win the championship."

Mills is not alone when it comes to trying to crack the Cubs' 26-man roster for the upcoming season. On the eve of games starting for the North Siders, here is what we know about the various roster battles that Ross, his coaches and the front office will be monitoring over the next month.

Fifth rotation spot
Top candidates: Adbert Alzolay, Tyler Chatwood, Mills

What we know
The favorite here is Chatwood, who put his '18 control woes behind him and pieced together a solid '19 campaign as a swing man on the pitching staff. The righty will be given every opportunity to take the final slot in the starting staff for the upcoming campaign.

Following a solid showing last year, Mills appears to be next on the depth chart. While Alzolay impressed in spurts with the Cubs in '19, he still only logged 81 2/3 innings on the year between the Minors and Majors. Chicago will be cognizant of that as it maps out this year’s plan for Alzolay. Colin Rea and Jharel Cotton are two additional options behind the leading group.

Second base
Top candidates: David Bote, Daniel Descalso, Nico Hoerner, Jason Kipnis

What we know
The Cubs could just hand the keys to the 22-year-old Hoerner, the team's top prospect who enjoyed a solid MLB debut in '19 and provides the best backup shortstop option for Javier Báez. That said, Chicago is also weighing whether Hoerner might benefit from more development time at Triple-A, considering he skipped that level and has only 375 career plate appearances in the Minor Leagues.

If the Cubs want to go with more experience, they have veteran options in Descalso (under contract) and Kipnis (non-roster). They both bat from the left side and could be paired with Bote (or Hoerner) to get the most out of matchups. That type of approach could also buy some development time for Hoerner before his inevitable promotion.

As games begin, there is no clear-cut favorite for the starting second base job.

Bullpen
Top candidates: Jason Adam, Rex Brothers, Danny Hultzen, Dillon Maples, Trevor Megill, Brandon Morrow, James Norwood, Tyler Olson, CD Pelham, Casey Sadler, Caleb Simpson, Brock Stewart, Ben Taylor, Ryan Tepera, Duane Underwood Jr., Brad Wieck, Dan Winkler

What we know
That is a long list, and it will remain lengthy until the Cubs start whittling down their spring roster in the coming weeks. Right now, the virtual-lock label can probably be applied to four arms: closer Craig Kimbrel, righty Jeremy Jeffress, lefty Kyle Ryan and righty Rowan Wick. From there, it will be a wide-open competition for the remaining four spots.

Left-hander Wieck made a big impression last season, so he might have a leg up on some of his southpaw counterparts. Mills, if he is not in the rotation, would also be a top contender for the kind of swing-man role Chatwood filled in '19. Underwood is also out of Minor League options, so a strong showing this spring could help position him for an Opening Day job.

Beyond that group, the Cubs signed Tepera and Winkler to Major League split contracts and brought Morrow back on a Minor League deal, given that both were effective setup men not too long ago. Tepera and Winkler each have a Minor League option, while Morrow is trying to use this spring to show he is over the injury troubles that have plagued him for past season and a half.

Other intriguing offseason additions include Adam (non-roster invitee), Megill (Rule 5 Draft), Olson (non-roster invitee), Pelham (waiver claim) and Sadler (via trade from the Dodgers). Internally, Maples and Norwood have flashed power arms and strong secondary stuff, making this an important spring for them to finally push for regular roles.

Bench
Top candidates: Carlos Asuaje, David Bote, Daniel Descalso, Robel Garcia, Hernán Pérez, Josh Phegley

What we know
For the outfield, Albert Almora Jr. and Steven Souza Jr. are virtual locks for the backup roles. Victor Caratini is set as the No. 2 catcher behind Willson Contreras. From there, the way Ross sets up his bench will depend in part on how the second base battle shakes out. Bote, Descalso, Garcia and Pérez can all be utility men, with Pérez offering a backup shortstop option.

For the 26th roster spot, Ross has said he is still determining the best approach for the Cubs' roster. That could include carrying a third catcher (Phegley) to remove some of the hesitance of leaning on the switch-hitting Caratini in pinch-hit situations. If Chicago wanted to use that 26th spot for a pinch-runner type, non-roster invitee Ian Miller could be a strong candidate.