First of all, I think your opinion is all you're going off of here. So let's get that out of the way quickly.
No, if he was "unknown" overall you're idea would be correct. If he were some guitar playing, pot smoking beach bum from the DR I would understand your blanket statement speculation.
This guy isn't unknown, he's just unknown to you, which has little bearing on his actual value. I imagine Daisuke was unknown to you two years ago as well, even though he had had two successful olympic stints, an amazing career in Japan and his Koishen performance and the fact that the Sox had been quietly scouting him for quite awhile. The crux of my 'theory' is not "all unknown players can be great", but that "players who have great success in a legitimate baseball league like Japan could probably contribute at a MLB level REGARDLESS of whether or not BoSox34 has heard of them or whether he makes outright claims about them not POSSIBLY being able to replace a mid-to-late thirties Manny if his contract expires."
I didn't see anyone saying he would be plugged into the middle of the 07 lineup. I saw people say he could play LF, but LF doesn't come with an automatic #4 spot in the lineup. Like it or not this team may need to replace Manny Ramirez with a player who is "only" the caliber of, say, Matt Holliday or Jason Bay. Just like they replaced one of the best pitchers of all time (Pedro) with someone who is "only" the caliber of David Cone or Mike Mussina in his prime (I think we've seen that level of upside already).