The other show many eyes were on — Two and a Half Men and Part 2 of its season opener — was expectedly down some 30 percent from last week’s all-time high (28.7 million viewers/10.7 demo rating). But make no mistake, Men is still outperforming Charlie Sheen’s unplanned February swan song episode, by 38 and 71 percent, respectively.

http://www.tvline.com/2011/09/ratings-terra-nova-hart-dixie-two-half-men/

The drop was expected to be 50%. This was great retention! The show will be fine. It'll return to its regular numbers by next week or the week after, and remain one of television's biggest comedies.