ST. LOUIS -- Now a week into baseball's free-agency period, teams are actively scouring outside markets for potential roster additions. But clubs are also making internal evaluations as a key offseason date approaches.

By Friday, teams will have to determine which players they want to protect from next month's Rule 5 Draft. Those eligible for that Draft include any player not on the 40-man roster who signed in 2012 at 18 years or younger, or in '13 at 19 years or older.

For the Cardinals, this year, more than any in recent memory, presents roster complexities related to the Rule 5 Draft. The Cardinals have three openings on their 40-man roster, yet they are likely to want to protect at least a half-dozen prospects before Friday's deadline. Thus, look for a few subtractions to be made within the coming days.

Remember, the club has already pared its roster down twice since the season ended. Mitch Harris , Alberto Rosario and Michael Ohlman came off in October. Dean Kiekhefer and Jeremy Hazelbaker were designated for assignment earlier this month. Both were claimed on waivers.

But the Cardinals understand how critical an exercise this is after having experienced both the gain and loss facilitated through the Rule 5 Draft last year.

The Mets' decision not to protect pitcher Matt Bowman a year ago left him available for the Cards to nab at the Winter Meetings. Bowman not only lasted on the Cardinals' Major League roster for all of 2016, but he emerged as a trusted late-inning arm.

On the other side, the Cardinals lost pitching prospect Luis Perdomo the same way. Gambling that he'd be unable to make the jump from High Class A to the Majors and stick on the roster all year, St. Louis left him unprotected. Perdomo is now with the Padres.

As the Cardinals prepare for their upcoming roster decisions, here's a look at players who will warrant consideration for protection:

The no-brainer: OF Magneuris Sierra

Sierra, ranked by MLBPipeline.com as the organization's No. 3 position-player prospect, will be added to the 40-man roster this week. This isn't up for debate. Sierra, 20, is exceptionally athletic and profiles as a future big-league center fielder. He hit .307/.335/.395 and swiped 31 bases with low Class A Peoria (Ill.) in 2016.

The middle infielders: Edmundo Sosa, Allen Cordoba, Juan Herrera, Eliezer Alvarez

This is as promising a group of young middle infielders as John Mozeliak has seen in his tenure as Cards general manager. Sosa, at No. 7 overall, is the highest ranked of this bunch on MLB Pipeline's organizational list. The 20-year-old played in 88 games with Peoria and slashed .270/.307/.343 on the season.

Herrera, 23, has climbed higher than the other three after reaching Double-A Springfield this season. But he also had the most difficult season, slashing .194/.234/.263 in 2016. In contrast, Alvarez, 22, hit .323/.404/.476 in 116 with low Class A Peoria. Cordoba, 20, is the only one of the bunch yet to play on a full-season Minor League team. That may sway the Cards to leave him unprotected given that he'd be less likely to stick on a big league roster for a full season.

The Arizona Fall League participants: RHP Rowan Wick, LHP Corey Littrell, LHP Ryan Sherriff

Last year, it was Kiekhefer who made a favorable impression in the AFL and parlayed that into a 40-man roster spot. Now, Wick, Littrell and Sherriff are trying to do the same. Sherriff and Littrell are both lefties, and that could entice the Cards, whose left-handed pitching depth is thin.

Wick, a converted catcher, is trying to make up for lost time. He began pitching in 2015 and posted a 2.64 ERA while splitting time between Palm Beach and Springfield in 2016.