Yes, it does take players time to adapt. I 100% agree and that's why nobody is expecting him to bag loads and set the world alight. These militant Kean supporters like to conflate the argument by insisting anyone criticising Kean clearly has too lofty expectations - it's simply not the case. As fans, all we want to see is a bit of energy, even if it doesn't result in a goal.
There are fundamentals we should expect of Football players whether you're in a new league or not, whether you're 17 or 37. Turning up on time and working hard are two of them. And that's where the issues with Kean start. His performances haven't been very good, his effort hasn't been particularly high and his attitude in response to being out of the team, hasn't been to knuckle down and prove his critics wrong, he's sulked and took to social media to complain. It doesn't bode well. He got a start in the cup against Watford and did absolutely nothing to merit it, he was rightly hooked at half-time and the game turned in our favour.
If you don't have discipline when managing a team, and you reward players who break the rules and punish those who follow the rules, you lose all respect from your team. That's why Tosun and DCL get the game time and this kid doesn't. It's nothing to do with raw Football ability, which Kean may well have more of than the two just mentioned - but unless he recognises that it's a team game, he'll never get the opportunities to thrive. He'll be another Balotelli, a waste.