A run of three losses in five games has seen the defending champions fall 15 points off the pace in the league, taking their tally of defeats to eight from 24 games.
United also face a fight to qualify for next season's Champions League, having slipped to seven points below the top four, and Rooney says that the time may have arrived to forget about the title.
"We know it's going to be extremely difficult to win the title," he told the BBC's Football Focus television program, broadcast on Saturday.
"Realistically, top four has got to be our aim. Although we always want to win the league title, maybe it's a bit too far."
It has been a chastening start to life at Old Trafford for United manager David Moyes, but Rooney spoke out in defence of the Scot, who launched the England striker's career at previous club Everton.
"David Moyes is a fantastic manager," Rooney said.
"I worked with him at Everton and for a number of months at United. He's done everything he can, but it's the players that have to do better and hopefully we can do that."
Rooney's contract is due to expire at the end of next season, but despite recent media reports that he is in talks over a new deal, the 28-year-old was reluctant to discuss his future.
"I've said all along I'm focusing on the football and will try to do as well as I can for the team and help the team be successful," he said.