Is it really so strange to use a back 4 if you don't have the players available to play the 352 formation? The only player we have who is truly suited to playing LWB is Rose imo, and of course he is suspended. Is playing a back 4 a bigger risk than playing a CB as a wingback? I'm not so sure. Vertonghen can cover at LB, but LWB is really pushing it.
As for Son and Lamela... well, Son is already needed as the striker if you play with two up top, but Lamela I really think is a better option than Vertonghen there. The best option with Rose out would actually be Dembele. Used to play on the wing, tracks back well, physically strong, surprisingly quick and is left-footed.