Systems. Johnson played as the main outlet in a deep block and counter side. Mitoma played in a system far more like ours. As we are seeing, in games where we are dominating territory and squeezing teams back, Johnson's straight line running is pretty moribund. Mitoma's 1 v 1 ability to dribble at/past people when faced up could be extremely useful tool, against deep blocks. he's also better without the ball than Johnson.
LOL