Don Pacifico its both. Xhaka wasn't all that great either, he just did the basics pretty well last season. Partey was definitely more important and his skill-set is too different to Rices. You can play Rice as your 6 but we'd need a different midfield in front of him to make it work. Right now its a lot easier to put Rice into Xhakas position and let Partey play his natural game. Rice is well suited to bombing up and down and is a better all round player than Xhaka at Xhakas role.
Our build up has been too slow this season. When Partey plays in there it speeds up our attack and allows Saka and Martinelli to get into danger areas before the opposition has reset defensively. Right now we are trying to suck teams into our 3rd but when we do and get the ball into midfield we delay again and they've time to get back, negating the positive effect of our risky passing around our 3rd. I'd much rather see Rice ahead of Partey waitng for the pass because he can make runs forward with the ball or lay it off to others.
If we want to keep Rice as our 6 then ESR should start because he will come short for the ball to Rice and when he gets it he has the natural inclination to move it forward quickly. We need to speed up our passing.