It could depend on if anyone is injured, who's hot/cold, pitcher vs. batter matchups, who needs a day off, or take out Lowell or Youk if it's a RHP. Obviously there's some work to be done, you're right about that, I'm arguing at 5 million/year I'll take Dunn regardless.
If it doesn't work out you'll probably have one of the better tradeable assets in the game in Dunn with a cheap contract. That should net us a huge prospect/package in return if things don't work.
My point is that I think it's too good of an oppotunity to say "it won't work, no bother trying" since it's cheap, doesn't cost us a draft pick, and can be tradeable if necessary.