Regardless of how Wenger was doing in other respects, the handling of Ox, including statements, were fine. He did show immediately we have something very special on our hands, with an already potent attacking threat, but at the same time a very raw player, with much missing in his ability to provide cover and obvious lacunas in his game overall.
It was thus a dilemma from the off - temptation to play him (particularly in desperate times) vs do the right thing, development, realistic assessment etc. And I think the boss both called it right, and followed that with words (which, as GM said, just continued on the same path now, even if you were really referring to earlier comments).
I agree it might seem a bit desperate to expect much from a player so young. It only rarely proves wise. But having said that, all the stuff mentioned above (which made the 'dilemma' in the first place) is what sometimes makes the exception. The few who do make it in like situations are similar to him in several relevant aspects: Talent bomb ready to explode - but not sure when exactly it has that crucial defining moment, confidence they can deliver and hunger to take it on, enough humility and maturity to tackle own weaknesses, plus some external factors (players in position missing or poor, etc.). Who knows. 18 year olds of lesser talent have made great impact before.