Tickets are still available for the friendly at CenturyLink Field on Wednesday.
The latest update from the team is that 64,000+ tickets have been sold, says the Seattle Times.
The stadium's full capacity is roughly 67,000.
United manager Sir Alex Ferguson said of soccer in Seattle: "Soccer is definitely working here. When we came here and played Celtic, of course, you could understand it. There was a great incentive to go forward from there, and now you’re seeing the fruits of that. "
On the game, Fergie added: "I think we need to stretch ourselves a little bit. Our experience of last year, MLS teams were very difficult in Philadelphia and Kansas – we lost a game in Kansas. It’s a good idea of how the strength of MLS is now. It’s improving, there’s no question about that. The crowds are improving, as has been indicated. It will be a tough game for us, but we need that."