The Reds host Ole Gunnar Solskjaer's at Anfield on Sunday, who they trail by three points on the Premier League table.
"When you're a top team, you only worry about yourselves," Reds legend Souness told Sky Sports.
"That's been the case at Liverpool the last few years, but this season is difficult. Liverpool are huffing and puffing, that is a fact, it's not me picking a dramatic statement out of the sky. They are not the same team, not finding the same consistency, but they sit second in the league.
"They will think that even with them not being at it, they are still second and very much in the fight. But if there's one game United want to win, or one game Liverpool want to win, this is the one. The rivalry is enormous and has always been there; they are two monstrous football clubs with monstrous successes behind them.
"I think for the first time Liverpool will go into the game nervous. For the last two years it has been non-stop plaudits for them, and correctly so because the football has been magnificent. But that has not been there this season, consistently. That's why they've dropped points you wouldn't have imagined them to.
"United are going there as a group of players feeling like they can get a result. If they do that, it adds to their momentum.
"If you'd have asked me at the start of the season if United would be facing Liverpool at this point of the season three points ahead, I would have said that isn't going to happen. But we're in the most unusual of unusual seasons that I can remember, but fair play to United, they've played with a dogged determination and crawled themselves right into the fight."