Tbh I don't really get the impression that Eriksen is exhausted, he was class for the first hour yesterday but isn't necessarily a footballer these days who can play at the same level for the full game, every game. His fitness is good though, and generally a Fred for Eriksen sub on the hour is a very good sub whether we want Fred to see out a game as he can press and help our counters, or help claw a game back as again, he can press and provide a lot more energy for us. He had a good 2-3 week break during the world cup as Denmark dropped out at the group stage, so he seems fine to me.
Long term, yeah of course, need a deep playmaker who has energy and can be press resistant. Frenkie/Enzo are both dreams for the midfield to partner Casemiro, but we are still in a good state now at CM IMO. I agree in big games, it's what Ten hag did vs City and I think he'll do the same vs Arsenal. I do think that's more down to just playing the highest performing 11, in which case Bruno gets pushed a bit to the wing because we have no attackers who are performing well apart from Rashford. I don't think he makes those changes necessarily if Sancho or Antony were in great form for example.