I've been as enthusiastic about Andrew McCutchen as any phan...so it pains me to say this, but...
Cutch will be playing at age 34 this season. That suggests further decline is possible... even probable.
Last season, his OBP, at .324, was below his career average (.376) by .050. That's not trivial. If that OBP does not rebound in 2021, McCutchen is no longer a premier leadoff hitter. For reference, Phillies who (in regular or semi-regular play) had higher OBP than Cutch in 2020: Harper, Bohm, Hoskins, Realmuto, Segura, Gregorius. In more limited play, add Knapp and Haseley to this list. The only hitters likely to be on the Opening Day roster who put up poorer OBP were Quinn and Kingery.
If Andrew McCutchen is not our leadoff hitter (because he's not getting it done), it makes little sense to keep him in the lineup in left field. He managed an OPS+ of 102 last year - that's really not left-fielder territory, in the absence of a very large OBP.
The complication? There's no obvious left fielder in the organization to replace Cutch. Haseley, Quinn, Moniak? I could see any one of these in CF, if they could deliver an adequate OBP to hit in the 2-hole (or the 8-hole). Haseley...might. Quinn or Moniak probably won't.
But if the top of the order is, say, Segura and Realmuto (or Segura and Hoskins), and we have a CF in the eight-hole, I would really like to see a bigger bat than Cutch in LF - or at the least, a LH-hitting OF who can put up an OPS+ of 110 or better in platoon duty. We don't have that player.