Be careful about confusing train and tram.
To get to the ground, the best way is a tram from Piccadilly station. The Old Trafford tram stop is right outside the cricket ground and it's a 5-10 minute walk to the football ground. If you don't want to walk it back to the city after the game, this...
Agree tram is the easiest way, but I would go from the station - not Piccadilly Gardens. You can buy a tram ticket on match days from sellers in front of the station. Piccadilly Gardens will be heaving and the trams already packed by the time they get there.
Going back, you should make it back...