I've got he Milan-United match on my PC and I've just watched it again. The real problem was Seedorf running off Fletcher, Heinze going walkabout at leftback and Vidic looking like he was running through treacle. Carrick's passing could have been better, but he didn't actually handle Kaka too badly when the two came up against each other.
Fletcher, Scholes and Carrick were occupied with Milan's midfield 3 and Kaka was actually one of their strikers. He found most of his space because of Heinze's suicidal defending and Vidic's lack of sharpness, rather than Carrick's inability to defend. If you watch the game again, Kaka (and indeed Oddo) did most of their work in the area of the pitch that Heinze should have been. That's where Milan won the game.
The other problem was how deep our midfield was playing and how much of a gap there was between them and the wingers, and between the wingers and Rooney. Whenever Ronaldo got the ball he had nobody to pass to, and judging by his generally selfish attitude during the match he probably wouldn't have passed even if he an option to. Gattuso ultimately had an easy job marking him out of the game as a result.
The 4-3-3 we played meant our midfield got pinned back, and we couldn't break out of our own half with any numbers. Hargreaves wouldn't have helped too much against Kaka imo. He may have had a better time against Seedorf than Fletcher did, but he wouldn't have been able to mark both Seedorf and Kaka at the same time. Especially since Kaka was playing up against our defence rather than our midfield. And Hargreaves' lack of passing ability obviously wouldn't have helped us relieve the pressure on the counter attack.
The thing that really killed us off on the night was the shite formation and 2 out of our 4 defenders barely showing up. 4-4-2 and and full defensive contingent is what we really needed, not Owen Hargreaves.