Name me a defender who was / is top class at 21?
Even exceptional talents like Stone and Varane are going to mistakes. Varane's naivety cost France the possibility of a place in a World Cup semi final.
I agree that Stones has been rash lately. Leaving Martinez with two options;
1. Drop him and let him work it out in the U21s / training. Debatable whether this works.
2. Play him and let him gain experience and (hopefully) iron out these mistakes.
Personally I'd go for option 2. It should enhance his value for when he is inevitably sold, it enhances the clubs reputation as somewhere that gives young talent a chance (obvious benefits) and we can watch him dribble around forwards and do cool stuff.
The only way I'd consider dropping him is in a crucial cup game, where his rashness may cost us, but is Mori less likely to make a mistake?