The problem with Foyth under this system is the same as it is under any system. He is not good enough to be a starter and is not better than Tanganga who offers the same skill set so how does he fit?
Maybe if we get two top level CB's maybe we could start one of them at RCB with those two and then have the other as the back-up but I just don't see the value of having both Foyth and Tanganga as they really seem to overlap each other.
Foyth is so good on the ball, by far and away the best ball playing cb at the club. Chelsea under Conte utilized Luiz as a ball playing cb who could sometimes go into midfield.
A big problem with our current squad is that we don't have many options who can receive and redistribute the ball well. I recall our game against Chelsea earlier in the season where they dominated us and won 1-0. Despite playing an extra defender to us they had way more players on the pitch who could receive a pass, redistribute and play one touch, this is something I feel Foyth would give us.
I agree that Tanganga is a promising player and even less error prone that Foyth but I think that Foyth would give the squad something different.