Nobody can say for sure that Lester will turn it around and nobody can provide any proof. There are, however, many, many examples of pitchers turning it around after a season like Lester just had. Keep in mind how young he is and how good he was for several years. He appears to have lost a bit of velocity, which is a concern, but he still throws reasonably hard. He may just need some 'tweaking' of his mechanics and now he will have his old pitching coach (Farrell) looking at him and a brand new pitching coach (Nieves) as well.
And others here are better at tackling these technical questions than I am.