When was he injured? Amad missed the very next game due to the suspension he got from the Liverpool game, then was back on the bench the next couple of games. The next time he actually got on the field against Bournemouth he looked impressive again, and he bounced between average to good in the remainder of his sub appearances for the next month (which was better than most of the other attackers at the time). I don't even have too much of an issue of that period when he was finally starting to get appearances off the bench; more how long it took for him to get those appearances and then again about how quickly ETH dropped him down the pecking order this season after just a couple of off-games.
At the end of the day, Amad was fit from January up until when ETH was fired at the end of October. In those 10 months he started just 10 games, and he was one of our best players, if not our best, in 6 of them (Arsenal and Newcastle in the league last season, City in the Community Shield, then Brighton, Liverpool and Southampton in the league this season). That's actually a fantastic ratio of good performances, so this idea that Amad really only kicked into gear after ETH left really isn't the case. He was already performing when he was actually given the opportunity, but ETH preferred numerous other players ahead of him.