MS doesn't need to lock up either of those games as exclusives. They just need to create a strong perception that new Xbox is the best place to play the new versions of the games you love.
If they show off new Live features, video sharing, smartglass, and illumiroom features within popular games like FIFA, CoD, Forza/PGR, etc. they'll create buzz among casuals and lock them up for another generation. That word of mouth is worth a lot, so I wouldn't be surprised if Microsoft has had a hand in helping developers extend their games with features Only on Xbox.