[gtvwill]

Send the protégé on some training courses and skills development workshops. Bring them up to speed with current trends and requirements of the business. If they kick up a stink or if questions are asked just make it out like it's the new norm. Then it's up to them to sink or swim. If they falter, point it out to the big boss. Mitigate failures by having paired programming where you have your top tier junior shadow the protégé. They can learn from each other, you can reduce potential damage that can occur from putting all your eggs in one basket.

Humans much like systems need redundancy too, never rely on one alone. What happens if your lead gets ill? Has a injury? Protégé or not you need redundancy.

Also talk to your boss. Communication is king.