I'm not going to pretend I wish the Sox did not sign him for slightly more, but $11-12M is still a lot of money. For a team looking to rebuild- mostly for 2022 and beyond, deals like this are just bridges and fan appeasement type choices and may take away from the ability to build for a stronger long term outlook. Yes, one year deals don't hurt as much when the player flops, but on the flip side, when they do very well, they leave a big hole the next winter.
Also, our rotation is full of pitchers with serious health issues. Kluber saw his velocity drop after 2016 from around 94 mph to about 92 mph. It's hard to be critical of his 2017 and 2018 seasons, where he put up some great numbers, including 2 of his best 3 ERA- seasons and, by far, his best two WHIP seasons (0.87 & 0.99), but if his velocity is now around 90 mph, there remains the big question on how effective he can be. If he needs to re-invent himself from a K pitcher or "thrower" to more of a finesse type pitcher, it may take more than a year, if ever.
Again, I'm not projecting doom for Kluber. I wish we had signed him to a 1 year deal, but if he struggles, it will be a big step down from Tanaka for the Yanks.