Why do you think he left Everton to join Saints? What do you think Saints told him and his agent in the summer? It wasn't to sit in our B team, like he would be if still at Everton.
Hasenhuttl talked about him in terms of the 1st team...
“We have already signed a player this summer with good experience at left-back in Romain Perraud, and Thierry’s arrival now gives us a very good balance and a strong additional option in that position, and this is encouraging that we now have this on both sides of our defence.”