I think your right saying Lamela is not a starter but i think its because of his goalscoring abilities not creativeness he had tied second most with 9 in the 15/16 season but Son and deli score way more goals and thats how you win games.
Alli and Dele are both massively creative and they could wrack up even more assists if they always passed but, in the case of Son in particular, there are many situations where he has the abilitiy to create a shooting opportunity due to his ability to shift it on either foot, which is hard to defend against, whereas Lamela, in the same situation, would look to pass because defenders find it easier to stop a shot when they know a player massively favours one foot over the other. And when we're playing a team with 10 men behind the ball, one of the things they do it shut down the passing lanes and it often takes a shot from the edge of the area to open up a game and opportunities to play a teammate in generally only come once we've gone ahead and forced the opposition to commit more players forward. This, in my opinion, is why Son should always start ahead of Lamela in games where teams are gonna park the bus, and also his ability to get into crossing positions because defenders are aware he can go either way, which is a massive headache for defenders, whereas with Lamela, defenders know to "show him outside" I.E. Allow him to go byline and attempt a cross with his right foot, and he rarely does this and generally will go back on his left and end up passing to a teammate. He did manage to get a cross in yesterday but it was just bad defending on Stoke's part giving him the space to do that instead of showing him outside, and it's not difficult to see why they have the poorest defence this season and Hughes will be getting his P45 soon.