Cross-threaded and preserved for posterity in its entirety.
David Ortiz is 32 years old. He's listed at 230 pounds; we've all seen him, and we know that 300 pounds is closer to the truth. He had serious knee trouble affecting his swing in April: 300 pound guys have knee trouble, trust me. He overcame that issue in May, and he damaged a tendon in the process.
We're talking about an injured slow-moving slugger who's lost his swing. That's not good.
Let's look at Big Papi's ten closest comparable players through age 31 (source Baseball Reference), and see when they had their "dropoff seasons"
Carlos Delgado 35
Mo Vaughn 31
Lance Berkman N/A (currently 32)
Fred McGriff 33
Jason Giambi 33
Richie Sexson 32
Jeff Bagwell 37
Willie McCovey 33
Kent Hrbek 32
Paul Konerko 32
Excepting Lance Berkman, who at 32 hasn't yet declined, let's look at a cumulative distribution on those ages of decline:
31: 11%
32: 44%
33: 78%
34: 78%
35: 89%
36: 89%
37: 100%
David Ortiz is 32. He a slow-moving slugger. Slow-moving sluggers decline while young. Most of his most comparable players dropped off significantly at either age 32 or age 33. Even before sustaining a very serious injury, David Ortiz was only hitting .252/.354/.486 this season. If he comes back at all, he'll be coming back mid-season without the benefit of recent regular play.
But you're suggesting that he'll hit .325/.409/.617 when he returns. I'm suggesting he'll continue near .252/.354/.486 IF he returns. Which is more likely?
There's a chart on David Ortiz's PECOTA Card called the "blood spatter diagram." Let's check it out:
http://www.baseballprospectus.com/pecota/images/ortizda01_004.png
The batting line for May was an Equivalent Average (EqA) of about .350. His batting line for the season thus far is an EqA of .279. The red splotches in the chart above show how well Big Papi's several dozen most-comparable players did in their age 32 seasons. Right now, if Ortiz didn't play again in 2008, his performance would be at (246, .279), right near the upper-left corner of that quadrilateral formed by four guys who collapsed at age 32. If Big Papi does come back, though, 500 PA is probably all that he'll get. The range of performance of comparable players to Big Papi who got roughly 500 PA at age 32 is an EqA of .280 to .340, weighted to the low side. What you're suggesting, an EqA of .350 or so, is rare for perfectly healthy players similar to Big Papi at his age. The median for healthy players looks to be about .320. That's lower than the .350+ EqA Big Papi posted last year, but that's normal: players decline with age. But the median for injured players like Big Papi is even lower. Expecting him to come back and to hit as he did in May is unrealistic.
There's room for more optimism than I offer; there's almost no chance that he'll be as good as you predict, though.
Which brings me back to that Chris Carter thing.
Chris Carter is having a good year, but not such a great year that we should chalk it up to luck. He hasn't regressed; in fact, he's getting a little bit better at age 25. Right now he's roughly at a .270 EqA if one converted his AAA stats to their MLEs. That's good, not great: it's roughly a Jacoby Ellsbury or Dustin Pedroia value level. Although Chris Carter is a very different player on the field than either of those two, his overall value at the plate might be about the same. It's MUCH better than Brandon Moss, though, who's posting an EqA below .230.
Putting Chris Carter into the lineup as a DH--in particular, as a platoon DH vs. RHP--gives Boston its best chance to win games and gives Chris Carter and the FO a chance to maximize Carter's potential trade value. Let's not pine and wax nostalgic about Big Papi's past greatness--let's get Carter some time in The Show to develop his skills and to show what he can do.