If we switch to a single pivot, it will change lots of dynamics. If roles change, performances might change. If we have two 8 working more for the offensive game, it might not be needed to have a midfielder "that can pull the strings" as long as the 8s are doing a good job. Plus: Remember who will play at Centerback for us.
I thought, Fred did the single pivot job pretty fine today. Better than I would have expected and he showed some shielding techniques I hadn't seen him use. I know, he isn't known to be a DM but he might fit the new role description unexpectedly well.
I can understand, that Matic is well liked and I agree, he might be useful for us but he is the past - just like Mata. Giving them gametime, in my eyes just takes minutes from other players either getting more used to a role or young players we would love to see their actual potential (like Garner). I know, Matic is currently well above them in a lot of relevant aspects but for me, playing him is pointless - he can be alright, today I thought he was fine but I would struggle to give him more than a 6.5 out of ten. That is good, no question but is it good enough that we delay working on the future in this part of the team?