When you think about it, we don't really play with a holding midfielder. We play with a double pivot, with Pogba on the left and our right winger dropping, which then effectively becomes a 4222.
Our first choice double pivot in Matic and Herrera is solid. Defensively very sound. However, the two of the them lack top class passing range.
As others have said, we need to be looking at someone who has the ability to control the game from deep.
We've recently been linked with Rakitic. Although he's 30, that profile of player would be perfect sitting next to Herrera/Matic, in my opinion. Players like Pjanic, Kroos and Verratti are also very good options and the kind of midfielder we should be targeting.
If i'm honest, though, I very much doubt we'll get a holding midfielder. Think we'll focus on getting an attacking minded midfielder to rotate with Pogba, but also someone who can play deeper.
As I said in other thread, someone like Lorenzo Pellegrini would be ideal as he could play in the 8 position, then as a 10 if/when Pogba is out.