Players can play multiple positions as emergency back up, but they shouldn't be double counted as the primary back up in multiple positions, which is what people keep doing in this LB debate (which
@Brightonian was correctly pointing out). Doing so leaves you with obvious weaknesses and a lack of proper depth.
You're allowed 25 senior players in a PL/CL squad which is typically made up of three GKs, two players for each outfield position, and then a couple extra. You can then supplement that with as many u21 players as you like. The positions we're reportedly prioritising this summer (CM x 2/3, LB, striker) is exactly in line with the positions where we no longer have two players for each position based on recent or impending departures:
RB - Dalot/Maz
CB - De Ligt/Maguire/Yoro (we have a bit extra at CB due to injury issues and younger players coming through)
CB - Martinez/Heaven
LB - Shaw/
???
CM - Mainoo/Santos
CM - Tielemans/
???
10 - Bruno/Mount
RW - Amad/Mbeumo
LW - Cunha/Dorgu
Striker - Sesko/
Zirkzee (or replacement)
Before he left Malacia was ostensibly our second LB, but was either injured and/or rubbish so Shaw played every game (and we're lucky we had such a light schedule and he miraculously managed to stay fit last season).
Having some versatility across the squad helps - it means during injury crises or for tactical reasons you can switch things about. But your primary cover should not come from the second options in other positions playing out of position. That's just poor squad building which makes your squad imbalanced and vulnerable, especially if you want to compete at the highest levels with a heavy game schedule of PL, CL plus domestic cup runs.
Mount and Bruno can step into CM if really needed, but that's not an adequate replacement for having standard cover at CM. Amad and Mbeumo can play on the left if really needed, but that's not in place of having standard cover at LW. Maz or Dalot can step into LB if really needed, but that shouldn't be in place of having standard cover at LB.
Now if the club saw Dorgu as a LB, it would make sense for us to go for a LW instead of a LB. But that doesn't seem to be the case, hence us reportedly being in the market for a LB.
The likes of Amass, Lacey, Fletchers and any other academy prospects who stay are third cover who can rotate in for cup games as well as providing emergency injury coverage.