NBA 2K12 keeps the Opponents stats somewhere. It has to, or else it wouldn't be able to calculate Metric Stats.
However, there's no screen in the game that shows them to you. Vlad Zola Jr, the creator of REditor, will be soon working on decoding the Team Stats from the 2K12 saves to add the according tab to REditor, and I've informed him of this, told him to look for Opponents stats too.
Until then, you can use NBA Stats Tracker, enter each box score after each game you play, and the tool will accumulate the Opponents' stats for you. You can check out both your and your Opponents' stats for any kind of timeframe, as long as you enter your Box Scores. And by Box Scores, I don't mean the players' stats as well. That's optional. You just need the teams' stats which are available from the View Box Score screen from the Calendar or Home Page.
To view Opponents' stats, go to Team Overview, choose your team, and then click on "Between" in the Timeframe part of the window (bottom-right). Pick the dates between which you want to see stats, and there you go.
To find out more about NBA Stats Tracker and download it:
http://forums.nba-live.com/viewtopic.php?f=143&t=84110