Team integrity vs new skillful engineer

I'm a manager of a small team (2 backend developers, 2 mobile developers, 1 tester, 1 product designer).

Apart from product designer, everyone else (5 people) are weak in the team. All my efforts explaining them value of writing tests, testable and readable code, things like SOLID and DRY – never worked. I skip details here, but believe me I tried a lot.

Fast forward, management finally hired new guy to my team. He is really good, within first week understood all the mess and proposed plan to improve.

We tried to slowly introduce his plan during last 2-3 months.

Problem is that rest of the team is defensive, and feels threatened by new skillful guy.

This skillful guy can't really make any progress because the unskilful rest is against and to be honest – they don't realize the mess and don't understand the fix proposals (but they don't admit to not understanding).

I as a manager have an impossible task of making it work. I can't change mentality of 5 people, I can't fire them, I can't risk loosing skillful guy (he told me he's frustrated already) and I can't ask skillful guy to give up and contribute bad code.

Anyone any hints or patterns how to proceed in such scenario?