Mt. Cody had his concerns about stamina, weight, and being a strict 2-gap DT when he was a potential 1st RD'er. Those concerns became reality when he showed up at the Senior Bowl weighing 370 lbs. He took a blow to his draft stock and is now a 2nd RD'er. He belongs no where near 1-gapper 4-3 DL, which is what the Titans run. Horrible fit. In fact, the main reason why he was originally valued so high was because he was a 3-4 NT prospect, which aren't too common. Since then, he's hurt himself, and other DT's like Dan Williams have stepped up and improved themselves. In fact, Dan Williams is probably the top 3-4 NT in this class now.
As for Williams to the Titans though, I don't like it and giving them a DT is about as lazy as giving them a WR prior to last year. Unless they can somehow trade up to the top of the draft and land Ndamukong Suh or Gerald McCoy, they aren't going to find a DT that'll replace Haynesworth. Plus, the Titans miss Haynesworth's pass-rushing with him collapsing the pocket and beating double teams, which is something Dan Williams won't do consistently since he's not an elite pass-rusher. Haynesworth also made the other players on the DL better, so without him, their play diminished and they played at the level that matches their actual ability.
So, the Titans are still left with the job of replacing what Haynesworth brought. They can either try to bring in a DT that makes everyone around him better, or they can upgrade the talent that needs an All-Pro DT to succeed. So unless they can somehow, someway, get Suh or McCoy, they need to improve that surrounding talent - most notably DE. Plus, the Titans have invested early picks in the DT position with Jason Jones (2nd RD'er) and SenDerrick Marks (2nd RD'er), and they have Tony Brown who is actually a very good DT (Yeah, he's not Haynesworth, but maybe 3 other DT's in the NFL are on Fat Al's level).
So yeah, love Dan Williams, but not for the Titans. Hopefully one of Morgan, JPP, or Haden falls to us. If not, for the love of little baby Jesus in a tuxedo shirt, trade back for Everson Griffen and don't draft Carlos Dunlap.