I honestly think Pervis Estupinan of Watford could be available for much lesser than what was quoted for Telles and Regulion. Not saying he is worldclass, but as a squad option with potential to be better and at age and price, is really a no brainer. Worst case is we sell him in a few years and breakeven.
Based on Opta stats from last season. The top 4 (green) are my outstanding candidates:
Alex Telles (10) - MotM, heading, assists, crosses, touch, possession, goals(*), key pass, shooting
Lucas Digne (9) - heading(*), assists(*), defending(**), key pass, touch
Vitaliy Mykolenko (7) - heading, avoid yellows, foul magnet, goals, long balls, crosses, passing,
Álex Grimaldo (6) - MotM, assists, avoid yellows, key pass, possession, shooting
Nuno Mendes (4) - through balls. avoid yellows, defending (*)
Sergio Reguilon (3) - dribbles, foul magnet, shooting
Renan Lodi (3) - defending (*), foul magnet
Pervis Estupiñán (2) - MotM, dribbles
Aaron Hickey (2) - dribbles, shooting
Fode Ballo-Toure (1) - possession
I took 11 candidates for left-back. Gave them points for stats they do well at. Occasionally they got 2 points (*), or 3 (**) when they do very well or stat is really important. Telles is top because he's a dead ball specialist so gets goals others can't. So
Mykolenko is my choice (he also scores on potential being one of the youngest). There are other LBs I didn't include because they're unobtainable or too expensive. I shouldn't have included Digne; he plays for Everton, who will charge us the earth.