For next year, there's a decision to make on who is the 2nd big bat behind Ortiz (who carries increasing downside risk each year)...
I think this line of thinking is misguided.
To state the problem of a need to replace Manny's/Ortiz's/whomever's production is to imply that the Red Sox need only and exactly so many power bats (or whatever). Should the Red Sox not sign top offensive players if they already have one (or two)? Should they start to focus on acquiring poor players if they have several very good ones?
Let's say player T, who is a GG-caliber 1st baseman, is offered in a trade for player Y, who is also a GG-caliber 1B (both are competent at 3B). Player T has a VORP* of say, 23. Player Y has a VORP of say 13.** So there is a difference of 10 (we could use any metric and any scale). Assuming cost is otherwise equal, why would you ever *not* make this trade? If the cost is not equal, or if there are other players involved, or money is a factor, then the values on each side of the scale become slightly more complex to tally, but the comparison is still the same. The player is ultimately either more or less valuable than what you are giving up to acquire him, factoring in risk, etc.
We can argue about the best way to measure likely output, or the best way to evaluate talent, but why would a smart team ever not
acquire a player whom they can afford and who improves their team more than the other available options at similar cost? And if a player does not improve the team more than the other available options at similar cost, why would a smart team not pursue those options instead? IOW, the only question that really matters is, after all the money and players and risk and opportunity costs and so on have been tallied, is this scenario the best available use of the team's resources? If the answer is no, then use the resources in whichever scenario was better. If the answer is yes, pull the trigger.
If Teixeira is good, and Manny is good, then both are better. If one is not good, then how does the absence of the other make him better? If they both played the same position, then I could see an either/or "fill the hole" mentality being relevant. But "power hitter" is not a position, and you can stock the lineup with them if you want to and are able to do so. Why stop after Ortiz?
My point is not to sign Teixeira or not to, just that I don't think that the quality of your left fielder should have much bearing on your options at 1B. If you have a hole at LF or a surplus at 1B or something like that, then sure, move to address it. But much like pitching, sluggers are something you can never have too many of.
Personally, while I think Teixeira might be an improvement, I don't think he is *enough* of an improvement to justify the cost. IOW, I think over the next few years, The Red Sox will be able to field a better team with with Youkilis and an extra ~$20m than with Teixeira. But Manny's presence or absence has nothing to do with that assessment. I just happen to think that Youkilis and Teixeira are more similar in terms of production than their reputations are, and that the money spent could buy more improvement at a position that is not already occupied by an All-Star-caliber player.
*I am using VORP because it is a handy bottom-line measure of offensive production, but we could use whatever value scale you want. My point is not limited to Tex/Youks, but rather about the big picture of how to evaluate signings and "plugging holes"
**In actuality, Youkilis currently has the higher VORP, 32.9. But for purposes of this illustration I'm assuming that Teixeira is better, to demonstrate a point.