Saints were down to 10 men within two minutes of kick-off after Alex Jankewitz was dismissed for a wild challenge on Scott McTominay.
The Red Devils took full advantage of the situation, but Ward-Prowse insists his team should not ponder on the result.
He said: "It's a difficult night for everybody. It was difficult from the start but we have to pick ourselves up from this disappointment and look to the weekend.
"We have a lot of games coming up and a chance at the weekend to show a reaction.
"We know how difficult [this situation] can be, but there's no point dwelling and thinking too much about it. We have to pick ourselves up and go again.
"It's a difficult game at the weekend and everyone is going to be looking at us now for that reaction and we have to go and show it.
"We have to be there and show up and show what we're about and show that reaction that's needed."