Ronald Araújo and Eric García are likely to start in central defence against Atlético Madrid, assuming Flick does not pick Gerard Martín.
The snag for Barcelona is that Flick has often deployed them out of position.
Araujo has started 33 matches across all competitions this term, yet he has recently operated as a right-back, even coming off the bench in that role during the first leg against Atlético.
Garcia, meanwhile, has been operating as a defensive midfielder recently, including in the first leg, so he too is rusty at centre-back.
With limited minutes alongside each other at the back, their partnership lacks rhythm and understanding.
That lack of understanding could be costly against an Atlético side that excels at quick transitions.