United boss Jose Mourinho launched an impassioned 12-minute defence of his record following the club's exit from Europe and then criticised his players after Saturday's 2-0 win over Brighton & Hove Albion in the FA Cup quarter-finals.
Lukaku, who has scored 25 goals this season, believes that more signings are now an inevitability despite Mourinho already spending £300m in less than two years at the club.
"We are Manchester United, we have to improve every year and I know that a lot of players, a lot of good players, will come in next summer. That's for sure," Lukaku told the Daily Mail.
"That is something I look forward to, in order to improve the squad, and what the manager also looks forward to because next year we have to win. We will have to try and win as many trophies as we can."
United are not in action again until March 31, when they host Swansea City in the Premier League.