The Red Devils lag 16 points behind their neighbours, who were crowned champions last weekend with a joint-record five games left to play.
United have been tipped to spend big once again this summer to bridge the gap, with Lazio's Sergej Milinkovic-Savic reportedly high on their list of targets, but Lukaku is sure that the current crop of players have it in them to learn from this campaign and go one better in 2018-19.
Asked what it will take to catch City, the Belgian told reporters: "I don't know, you'd have to ask the manager. I'm really confident in my teammates. I think we can do it with the team we have now, but we should have showed more consistency throughout the season.
"For us now it is up to us to be more consistent. The manager needs to see if he has to add more players but with the players we have now, I think we have enough. We just have to be more consistent."
United advanced through to the final of the FA Cup on Saturday evening with a 2-1 win over Tottenham Hotspur.