The most natural replacement would be Cohen Bramall, who is an actual left fullback, who spent last year on loan with Birmingham City. Outside of that, it’s playing players out of position. I would much rather see Arsenal play a specialist left fullback in the left fullback position as opposed to continue on Wenger’s round peg in square hole approach to squad management.
Either way, with the historically poor defensive production of Arsenal’s central midfield (Torriera aside) and right fullback options, I think it’s unrealistic to expect so many of the same players (e.g. Bellerin, Lichtsteiner, Xhaka, Ramsey, Elneny) to all of a sudden demonstrate defensive capabilities that they have not done so throughout their professional careers (even with Emery’s influence).
For that reason alone I’m not expecting Arsenal to get anything from their clashes versus Man City or Chelsea, irrespective of who plays at fullback.