A lot of amateurs don't deal well with pressure. Thing that stood out to me was that Joshua was not using his Jab aggressively as part of his offense and defense. Nistor made it his fight and the kid tried to bang with him here or there just to get some breathing room rather that using his distance, ring generalship and clinching to slow down the pace of the fight. I haven't followed Joshua after that but if hes made his jab a better part of his arsenal and polished his footwork and learned to clinch. He should be fine. Just the trial by fire of a fighter. Its better that it happened to him early, rather than him being spoonfed in a gym till he's 23 and then getting thrown to the wolves and becoming a professional opponent.