Repairing Polybutylene leak
If your only leaking area is what you see, then I would cut out only what you see and replace it. The problem with just replacing the "T" is that you will have to cut the pipes and that means shorter pipes and more adapters....more leaks later on possibly. I would cut about 3 inches from the right side that you see in the picture and as close and possible to the top of the "T" and remove all the stuff on the left all the way to the copper pipe. Replace all that with PEX. Or just cut your "T" out and shark-bite it.