Foden couldn't do what Saka does ... brilliant player, great touch and a precise marksman, but his game relies on covering a lot more ground and entering broken play in a little space.
Saka is the guy whose game is known inside and out by the opposition, so they've got a couple of defenders instructed to stop him, and he's generally got little breathing room, but he still does it anyway. Week after week we're able to run matches either directly through Saka's threat, or alongside because of it.