Agreed, one game at a time. That doesn't necessarily contradict thinking Sunday is more important.
For me, the league has to take precedence. The difference between this and (most) other seasons is that we've gone from seeing this - at least the group stage - as a chance to give reserves a runout, to tinkering a bit with rotation. I don't want N'jie, Wimmer, Trippier or anyone else to start just because. Likewise I don't want Kane, Toby or Walker to be rested just because.
End of the day, we should play the strongest 11, but looking at replacing any "league starter" with a squad player, it's gotta be a consideration of two factors. How much does the starter need a rest to be perfectly fit for the League, and how much does the squad player need a game to be happy/feel involved with the team, to continue his development and to retain some match sharpness if and when they're needed due to injury. I think that's how Poch is weighing it up this season as well, looking at how we've started other cup games. As such, it's near impossible to have an informed opinion on how we should start these games, as we don't know those factors.