You're right. Sacks are the be-all, end-all measurement of an OL and the OL this year is much better than last year's.
Or you could take a look at more than that. This year, the coaching staff finally recognized that C and RG were an issue and made moves to correct the issue. Then, for no reason whatsoever, jettisoned it's best interior OL. What we've seen as a ripple effect: Solder suddenly struggling mightily with pressure continuing to come up the A and B gaps as well as on the edges due to the fact that Mankins is no longer there to bail them out. Why do you think Cannon looks as bad as he does as well? Mankins was the one thing holding that group together.
Further, you haven't seen in recent years (and this includes 2013), the team intentionally installing a run heavy game plan to save Brady from getting hit when he drops back and the pocket predictably collapses. We saw that in Week 2. In Week 3, we had to air it out because the OL couldn't open a hole for Ridley or Vereen to save their own lives, and the result was Brady getting up after obviously getting shaken up on a hit by Justin Tuck (courtesy of our new LG) and another hit where Brady got up shaking his head at Devey. You're more than welcome to tell me the last time Brady showed any sort of level of disgust at his OL.
Basically, the team downgraded at RG (as hard as that is to believe... but Devey is terrible), remained the same at C until Stork starts, and downgraded at LG. So, as a whole, the interior OL got worse and the biggest reason for it is the loss of Mankins. They'd still be bad with him there because, as he has shown since 2012 to anyone with a semblance of knowledge about OL play, he can only cover up for so much. But at least the Pats wouldn't have to restrict their passing offense so their 37 year old quarterback doesn't absorb nearly as much punishment.
Solder's play didn't slip as badly as it has all by itself. Mankins getting traded is getting Solder exposed now. But yes, Solder has never been the franchise LT that many here have made him out to be, unfortunately.