Missing the relevance here?
If anything, Middlebrooks is the cautionary tale. He was a fan favorite right out of the gate, but turned into a massive flop rather quickly. (But at least Devers doesn't have Middlebrooks' noticeable poor strike zone judgement. They are not the same player at the plate.)
Regardless, that Sandoval and Middlebrooks were not good players doesn't make Devers a better defender. If anything, he looks like a hitter who is given defensive pass because of his bat, while other players who excel at their positions are crucified because of their bats. Defense is about the glove and the arm and using them to make plays. To date, that has not been Devers forte. On the bright side, even if he does move off third, he will still be a bat in the middle of the Sox lineup for 5 more years...