Milan would be more than happy to switch Mexes with Leo. Mexes is a shitty physically-strong defender with low levels of football IQ, he's more experienced than Leo but still, if he's the one who plays for us, he'll cost us more goals than the amount that Leo did.
Leo has years to come to grow as one of the best Italian defender, his mistake will make him stronger as he'll learn from it.
And no, Im not biased, I know Leo can be frustrating at times, but allowing he plays regularly despite the errors will be a great investment for us, and for Leo himself.