I don't think you have a good idea of what happened in this case. Mourinho wasn't impatient with De Bruyne, it was the other way around. The latter was 20 years old at the time and Chelsea treated him as such. He was told to be patient and that he'd get more and more playing time but he wouldn't have it. Jose didn't think he was ready to play regularly yet, especially since KDB was reportedly shit in training, unmotivated, crying and complaining all the time. KDB felt he was not a normal young player and was experienced enough to play regularly. Apparently he needed to play every week, in order to be motivated in training.
I don't think Mou was doubting his talent, just his mentality and dedication. It's possible that he made a mistake in that regard but it's also possible that KDB was indeed a whining little brat, at the time.