The Boston Red Sox need to learn from their past mistakes. Not signing Jon Lester before the 2014 season is one of the biggest screw-ups of the John Henry and Tom Werner era.
Chris Sale has proven that he can win in Boston. Sure, he has some durability issues. But pitching is what wins you championships. The alternative to not signing Sale and risking losing him to free agency like Lester is a far worse scenario than signing him and dealing with injuries.
The injuries Sale has had have been manageable.