I can't see Liverpool paying anywhere close to £20m for a reserve left back. They got Robertson for £8m a few years ago and Covid will affect values.
Lewis can get up and down like Robertson, but he doesn't have the same attacking attributes and his final delivery isn't anywhere near as good. They both have a weakness where the ball is played inside them, as neither can use their right foot.
But maybe this has legs. £10m, including a player swap and some add ons might not be such a terrible deal, with McCullum already in the building.