Haggis has a good plan! Check on parking though as at least some of Navigation Road is parking permit only for residents (I have a couple of friends who live on it). There are pubs around but I think people are getting more vigilant especially trying to stop commuters taking up spaces from residents and customers.
The tram stops right near the ground (just a few mins walk) and (providing the service is running ok - you can never tell with Metrolink!) you can get a tram from the station to Old Trafford (Altrincham line).
The met (trams) can get very busy though around big concerts. I live in Manchester and went up to see Take That and have never seen anything like it and I've seen some massive queues for football matches and when Oasis were there etc...! The trams were only running from a certain point then though while they were doing up the city centre lines but I had to queue for well over an hour then it was squashed on like sardines.
Luckily I lived near enought OT to walk home to Salford Quays and get a friend on a tram there to help him get home quicker. Different tram line though and mainly resident only parking so wouldn't recommend anything like that.
The traffic was ridiculous too, a friend tried to drop me off from town one of the nights and it was horrendous, way worse than during Utd games and that's busy enough.