Mirth How often do you see Salah supporting Alexander Arnold in defence? Saka's starting position I believe is also significantly farther back when we counter. That's what I mean by different roles.
I went to squawka to check on a couple of defensive stats for both players. https://www.squawka.com/en/comparison-matrix/
Tackles made, take ons completed, take on success %, fouls won, clearances, interceptions. Saka is head and shoulders above Salah as an all round footballer. Salah is basically an end product machine in the mold of Ronaldo. He and Saka, on paper, might play in the same position, but the way the position is interpreted by the player and what is required by the team from each of them is very different.