You've got to factor in how many game we're playing. Pep is the master of innovation, and he loves his players to be multifunctional. Sane could theoretically play three different positions for us.
We need the options and players who each bring a different skillset, what Sane may struggle at, Navas may excel at. You can be certain that if we're willing to spend £40m on Leroy Sane then he's certainly going to start a lot of games.
But will he start more games than at Schalke. When it comes to development he'll most likely get the most first team time at Schalke unless their are injuries. The move would be similar to Coman who came much cheaper from Juve and played significant time and started plenty of matches when Robben/Ribery weren't fit. When they came back he was the first attacker off the bench.