Having a draw in their last match by Manchester united is still a good result, and shouldn't be used to judge their match with Barca. EPL is different and tougher than Laliga,which gives Manchester united more advantage over Barca. It will be a difficult and interesting match,since it has been long since this two clubs clashed.
Manchester United and Barca are performing better in their various leagues. We have seen how both teams have been winning their matches lately. Manchester United drew to Crystal Palace and Barca won over Real Madrid in the super cup. Both team's last matches can be used to judge the upcoming match that will happen between them in 3 weeks. Barca and Manchester United do have other matches to play with other clubs before then. Maybe before then, the situation of injuries or loss form might occur between Barca or Manchester United. Which might be an advantage to the other team