The Premier League giants played out a lacklustre 0-0 draw at Selhurst Park and lost more ground on Manchester City, who are at the top of the league table.
Manchester United have won just three of their last eight league games and have struggled in the title race since going to the top of the league table in January.
They played a second goalless draw in a row at Crystal Palace and McIlroy admitted that Manchester United never put the home side under any sort of sustained pressure throughout the game.
He conceded that they did not show enough quality in the final third and believes the Crystal Palace goalkeeper had a relatively comfortable night out against Ole Gunnar Solskjaer’s men.
McIlroy said on MUTV after the match: “I don’t think we did enough [to win the game].
“I don’t think we put Palace under enough pressure, I don’t think the quality was there in the final third right throughout the game.
“There were a couple of incidents where we could have done better, but we didn’t make their goalkeeper work as hard as I expected to.
“And the all-round performance, I think the manager is a little bit disappointed with as that was a game tonight, for me, we should have won.”
Manchester United still have a seven-point lead over teams outside the top four but have played a game more than Chelsea in fifth.