It is true that Lewen has been out of position so far when he has played. In fact crazy to stick a young inexperienced striker out wide and expect him to perform to premier league standard at the same time.
Personally, i believe that he is nowhere near ready for the first team but his inclusion in the sides has not been a marker for how much Koeman rates him, more of a marker of how little he rates Valencia and Kone. This lad is starting games before them two, Koeman clearly doesn't fancy either of his experienced back up and with Bolasie/Lennon/Del all out in terms of wingers plus lookman having issues, he is throwing Lewen in at the deep end hoping he will do something of note.
I don't believe he will be an active part of the team next season simply so he can learn the game and get some experience whether it is in the academy or elsewhere. Right now, he is simply just a message of how little chance Kone has of getting a game.
In fairness mate, most strikers start out wide when they first come into the team if they are in the mould of Lewin. It's not really crazy, it's a good way to get them bedded in. At times (like vs Hull) it worked, and others (vs Swansea) it hasn't.
But yeh, it's a case of sink or swim and DCL has definitely kept his head above water.