Ted Williams only appeared in 76.5% of Red Sox games after he returned from the Korean War but he managed OPS+ of 201, 209, 172, 233, 179, 114, 190.
Wait, so you're saying Ted Williams was a better player than Dustin Pedroia?
Seriously, though, I do see two caveats to chrisfont9's point that "it's probably more reasonable to assume that he'll perform close to his old self than not" if he can manage this improbable comeback.
One is that it's inherently overoptimistic to expect a 36-year-old to match his performance level from several years before. This is true even when said 36-year-old is not coming off a two-year layoff due to a serious injury.
The other is that when we talk about "his old self" we may be talking about two different people. One is peak Pedroia, ages 24-27, who put up a 122 OPS+ while averaging 20 steals a year at an 80%+ success rate. He was a legitimately outstanding offensive player. The other is post-peak Pedroia, ages 28-33, with a 110 OPS+ and half the steals at a lower success rate. Still a clearly above-average hitter, but with much less power and slightly less eye-popping discipline/contact skills.
What I think would be a realistic expectation (if anything regarding a Pedroia comeback can be called realistic) for his bat would be a considerably lesser version of that post-peak guy, reflecting his age and two years off. We can also expect his defense to be diminished. He'll never not be a skilled second baseman, but we have to assume his mobility will be sharply curtailed. Likewise on the basepaths.
So basically I think we're talking about a guy with something like a .280/.340/.380 slash line, station-to-station baserunning, and defense that's competent but limited in range. And I think that's the
upside.