Foden's only been available for half of the season due to injury. He was completely out of the first 4 games and was limited to a 15-20 minute sub appearance on his first game back from injury. Trying to compare his minutes to Sancho's is really disingenuous.
And of course Ronaldo's impacted his potential game time. Without Ronaldo Utd have 5 players for the central and two wide forward roles, with Ronaldo it effectively becomes 4 players for the two wide forward roles as Ronaldo's playing almost every game through the middle.