For the return leg, it's crucial that Flick gets his lineup spot on. For me, that means absolutely just ONE of Rashford or Lewandowski MAX. Otherwise our press is non-existent (it's already suffering because of Raphinha absence obviously). Fermin instead of Olmo (no brainer in that #10 role at least) should also help our press some, but the most important thing for me would be, with Balde back I would LOVE to see Cancelo, one of the best players to play against a parked bus, start at LW (I think this is a crucial call Flick has to make, if we're to have any real chance of turning this around), with Fermin, Yamal and Lewandowski. I would only play Olmo later in the match (he seems to have a better impact when coming off the bench anyway), but only to replace Lewandowski and go with the false nine option, if/when Lewa can't go on.
De Jong (and Bernal) hopefully back, we need a Pedri-De Jong masterclass in midfield. With Bernal also playing a part, since both Pedri and De Jong might struggle with fitness at some point. Also crucial so that Garcia doesn't have to play there, so he can replace Cubarsi in defense.
So I'd go with a lineup of
GK: Joan
RB: Kounde (even though I don't like him, especially right now, but still better than risking Araujo in a big CL game again).
RCB: Garcia
LCB: Martin (same thing as with Kounde, with the bonus that I don't think Martin can bounce back and improve to the point of ever being good enough, but still, in this situation, better than risking Araujo)
LB: Balde
RCM: De Jong (I would absolutely look to bring in Bernal at some point, if he'll also be back and especially if De Jong won't be ready for the whole match)
LCM: Pedri (let's hope the rumours regarding the 'precaution sub by Flick are true, cause we need our magician desperately)
RW: Yamal
CAM: Fermin
LW: Cancelo
ST: Lewandowski.
Bernal and Olmo to come on at some point as subs. And possibly, depending on the game state, also Rashford, Ferran and Gavi, who needs to get at least 60, if not 90 minutes (replacing Pedri in that game obviously) against Espanyol to get into the groove and then on Tuesday I could maybe see him helping with our intensity in the last 20 minutes (as for him in a more overall sense, I would look to play him primarily in La Liga until the end of this season to slowly get back to full fitness, but he could still be a limited time asset against Atletico too).