Nothing to do with Phil Foden, Jack Grealish or Uncle Tom Cobley. The only thing that's changed is that we don't have a striker to provide a focal point / distract defenders away from Raheem. It's about movement - We've changed our attacking style to accommodate for not having a striker.
Raheem clearly has a problem fitting his natural game into the 'false nine' system, not least the fact that it seems to make it easier for teams to 'double up' on our wide players. Unlike our right side, Raheem doesn't have the benefit of a Kyle Walker type to use behind/alongside/ahead of him. Mendy was supposed to be that player, but instead he has a right back (Cancelo) and a midfielder (Zinchenko) - neither of whom fully 'fit the bill'.
Raheem isn't the one to blame - He's trying to adapt, but it's not been working. Pep clearly hasn't given up on him, so he must think it will 'click' one day. I think we'll see him back to his best when we sign a traditional striker.
You play your best players to their strengths - As long as we don't have a striker, Raheem will continue to struggle to make the same impact. If we don't sign a striker in January, a six-month loan to Barcelona would probably do him a world of good. Whether he'd want to come back after a spell in a better climate is another matter!