Abreu was more valuable last year, albeit in a Covid season cut way short of games and he had 40 extra plate appearances than Anderson did, but what he did offensively in that shorter season was tremendous. Defensively I'd say they were even at their respective positions, but shortstop is always the more important of the two versus first base.
This year, Moncada has been their best position player by a hair over Anderson because of more plate appearances and slightly better defense. His defense at third as actually been quite outstanding. Anderson has been very good, but Moncada has been better. Moncada has also improved his plate discipline a great deal this year. A career high 80 walks. Anderson has 20. At the top of the lineup. Speed and athleticism do nothing if you're trotting back to the dugout after an out. Moncada's slight edge in the metrics is likely due to this.
Anderson doesn't walk a lot, so if he's not getting hits, he's not doing anything. That's a bit of an issue, so I would agree that if he's not doing anything it's a problem. At this stage of the game, this is just who he is and it's not going to change. It just seems odd that in this era they'd keep hitting him first when he has no intention of taking pitches. Then again, dudes like that are a pain in the ass to get out.
Bottom line, if everything as is stays the same, it's a problem. If you get Eloy Jimenez hitting and getting on base, you advance. If not, I'd bet on the field.