Ole Gunnar Solskjaer said Manchester United will "take some time" to close the gap to Manchester City and Liverpool after they finished their season with a dismal 2-0 defeat at home by already-relegated Cardiff.
United finished in sixth place in the Premier League, 32 points behind champions City, who accumulated 98 points, edging out Liverpool by a single point.
That gap between Solskjaer's side and City is wider than the difference of 30 points between United and top-flight winners Leeds in 1973/74, a season when the Old Trafford club were relegated -- albeit in an era when teams were awarded just two points for a win instead of the current three.
United finished five points behind fourth-placed Tottenham Hotspur, meaning they miss out on qualifying for the lucrative Champions League, which will make it harder to attract top-class players to the club.
"Today was not the problem," said United manager Solskjaer, who previously had a brief spell in charge of Cardiff, after away fans at Old Trafford on Sunday had chanted he would be "sacked in the morning".
"You can have all the possession and chances you want but we concede easy goals and not score at the other. That is not where our challenge lies -- we have a long and hard way to be where we want to be."
The Norwegian, who initially oversaw a dramatic upturn in form after he took over from Jose Mourinho in December, said: "It will take some time to close in on the top. We finished five or six points behind third, fourth, fifth and that has to be our aim.
"The top two teams have set a standard higher than before and we have to take up the next challenge."
The former United striker added: "The work starts now. Everyone has the summer off but we have to come back with a different mentality and different attitude of being a Man United team.
"The ones who played, did well but it was not good enough."
United last won the Premier League title in 2013, in Alex Ferguson's last season in charge at Old Trafford.
Since then they have finished seventh, fourth, fifth, sixth, second and now sixth again -- a far cry from their repeated title triumphs under the outstanding Scottish manager.
They won 14 out of their first 17 games after Solskjaer replaced Mourinho, including a remarkable 3-1 triumph at Paris Saint-Germain to reach the Champions League quarter-finals but have lost eight out of 12 in all competitions since then.
'Back to where we belong'
Solskjaer said the past couple of months had been "extremely hard" for the club.
"More or less the only positive from today is that the season is over," he said following boos from home fans at full-time as United ended the campaign without a win in six matches in all competitions, and just two victories from 12 games.
"We meet again 1 July and we'll work back to where we belong.
"To reach the leaders it will take a long time. For ourselves the short-term focus in the league will be getting into the top four.
"We've got the Europa League, FA Cup, Carabao (League) Cup to go for a trophy because Manchester United can't go many years without winning a trophy.
"Jose (Mourinho) won the Europa League last (the last time United won a trophy, in 2017) and that is a realistic ambition."
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