The Reds’ campaign has stalled with just one win in their last seven games in all competitions.
Jurgen Klopp’s side are already playing catch up on their Premier League title rivals following a frustrating draw with Newcastle on Sunday.
They now have to wait until after the international break to get back on track.
But Kop stopper Mignolet believes the visit of United to Anfield on October 14 could be the perfect game for them to get back firing if they can get a win over their rivals.
He said: “It’s disappointing and frustrating but we have to keep looking forward, keep our heads up.
“We have to make sure when we come back after the international break we go again against Manchester United, which is a big game and something to turn things right for us.
“Everybody will be away in another environment and when they come back we’ve got that one to look forward to.
“I’m sure everybody will be focused on their international duties now but when they are back they have a big game to look forward to. We will be ready for it.”
The Reds are seven points behind United and City heading into this international break with three wins, three draws and one defeat in seven league games so far.
They wasted a number of chances against the Magpies on Sunday and another defensive lapse also cost them at the back for Joselu’s equaliser following Philippe Coutinho’s stunning opener.
Liverpool had 17 shots but only two on target against Rafa Benitez’s men.
Mignolet added: “At one point these balls will fall for us and we might score a lot of goals. At this moment in time, it is how it is.
“It’s not that we’re not playing well. I think we’re still playing good football and creating chances. We have to capitalise on them and be more efficient.”
