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 edged out Liverpool by a single point.
They were five points behind fourth-placed Tottenham Hotspur.
"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 don't score at the other end. 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."