We have no territorial control over matches when Tosun starts. He's not quick enough to trouble defenders chasing balls into the channels, he's not strong enough in the air to win enough headers for us to play off of, and he takes up a lot of the same (bad) positions as Sigurdsson.
There's a reason he's barely started since the beginning of the season. Calvert-Lewin and Richarlison have major shortcomings as CFs but the way they harass defenders and chase lost causes at least earns us some opportunities in the final third. Tosun, on the other hand, is a passenger.
Playing a 4-2-4 with Sigurdsson and Tosun in the team is like playing a 4-2-2 (with the ball, at least). Everyone else has to do a ton of heavy lifting to get them on the ball in the crap positions they take up and we've seen over and over that we don't have the players to make that work.