It's honestly not surprising to see Tek struggling at the plate. He was never an elite hitter, just good. And considering his age and how much wear and tear is on his body, he's bound to lose bat speed. And when that happens at the ML level, your days as a hitter are numbered.
Also, I don't think the Red Sox brought him back for his bat. I think they where well aware he could be aiming for the Mendoza line this year.
For a comparison, look at Pudge. He was a much better bat then Tek ever was. But he's up around the same age and losing bat speed. And his numbers have shown it. Once a Catcher hits 35, you can't expect them to be able to produce the same anymore. Your best hope is they can stabilize their down turn in production, and hope it falls slowly instead of rapidly.