Yeah, Seghir will definitely stay as starter, Fati isn't good enough to take his spot. The plan here is more likely to give Seghir more rest in the league, since they play also in the CL.
Fati will get more minutes than with us obviously, but I also think that's not what he needs. The french league is a good test for him in general to see, if he can compete on this level or should better look for teams in Portugal, Netherlands, Belgium, Turkey or Arabia.
But he should definitely find a team where he'll most likely start to get rhythm and confidence back. Though I think his stubborn character will keep telling him to go to a CL team to keep sitting on the bench.