That is United's problem, they never build on good results. They are playing Arsenal in the Fa Cup in a few days, we would see if they would build on their performance against Liverpool and go on to beat Arsenal.
I think the match will be extremely competitive for both teams, given their current performances. Although Arsenal has the home advantage, Manchester United's recent form, as seen in their match against Liverpool, suggests they might hold Arsenal to a draw after 90 minutes.