To be fair - when Son goes, I'd love for us to move to a back three temporarily until he's back. It get's the best out of our wing backs which add to our attack with Son gone. Gives us a bit more strength in defence and also allows us to rest the likes of Winks or Sissoko without feeling shallow defensively.
Still not sold on Lucas to be honest - think he'd be much more useful coming off the bench anyway.
Lucas has nabbed a few goals granted but I'm with you. He seems to be a little off the pace of the English game, not artful in possession and seems a little too direct at times when the clever pass that would be better seems lacking. Play him against maybe 13-14 of the Prem clubs but he can't be starting against the better teams in my opinion