Louis van Gaal's side will be looking for their third consecutive win in the Premier League when they travel to the Hawthorns, while the Baggies will be trying to bounce back from defeat to Liverpool prior to the international break.
Albion midfielder Morrison told the club's website: "I don't think that there is a right time to play Manchester United. They have picked up a few good wins recently so it is going to be a very tough game.
"They have got some fantastic players. They are top professionals and their manager seems to be really strict. I don't expect them to take their foot off the gas at all. I expect to come up against a fully prepared Manchester United."
West Brom are currently 14th in the table, while United sit three points above them in fourth.