1B/DH/OF Alec Burleson
Yes, I know Alec Burleson doesn't play third base. How is he affected by Arenado staying?
Well, Arenado playing at third base could push Nolan Gorman to second base. This moves Brendan Donovan to left field and Lars Nootbaar to center field. In this alignment, Burleson is left being the team's primary designated hitter. This isn't a bad thing for Burleson's development; he'll still see around 500 plate appearances as a platoon DH versus right-handed pitchers.
If manager Oli Marmol opts for a more defensive-minded lineup, that puts Brendan Donovan at second base, Lars Nootbaar in left field, and Michael Siani or Victor Scott II in center field. Nolan Gorman will then shift to designated hitter duties. Now, Burleson is left being a left-handed bench bat.
INF Thomas Saggese
Thomas Saggese may be the most direct player who is impacted by Nolan Arenado's decision to stay in St. Louis this year. Saggese has found his home in the infield, particularly at second and third base. In fact, he's been taking reps at third with Nolan Arenado himself this spring.
However, that may be the closest that Saggese gets to the major-league roster until someone gets hurt or Arenado is traded.
With Nolan Gorman shifting to second base more than likely and Masyn Winn getting the most run possible at shortstop, there isn't a real need for Saggese. Also, for his own development, he needs to be getting regular at-bats. The only real place he could do that in 2025 would be in Memphis.
Thomas Saggese is a key part of the next wave of position players that includes JJ Wetherholt, Chase Davis, Victor Scott II, and Jimmy Crooks. If he can't get regular reps in the majors, he needs to see those at-bats in the minors to maintain consistency. He absolutely obliterated the ball in the Arizona Fall League; that trend needs to continue regardless of where he plays this year.
If an injury occurs to any one of Brendan Donovan, Nolan Gorman, Nolan Arenado, or Masyn Winn, Saggese's name will be the first one called to fill in.