The Red Devils lost 1-0 to Chelsea FC in the FA Cup final at Wembley at the weekend after Eden Hazard scored a first-half winner from the penalty spot.
Manchester United finished without a trophy in the current campaign following their disappointing defeat by bitter rivals Chelsea FC in the Wembley showpiece.
The 20-time English champions finished in second place in the Premier League table ahead of Tottenham Hotspur, Liverpool FC, Chelsea FC and Arsenal.
However, Manchester United ended up 19 points adrift of Premier League champions Manchester City to highlight the widening gap between the derby rivals.
Manchester City, Liverpool FC and Tottenham scored more Premier League goals than Manchester United to raise questions about their style of play under Mourinho.
Former Newcastle striker Shearer believes Manchester United need some new signings and a new mindset ahead of the new season.
“United do not look like they are anywhere near champions Manchester City, who finished 19 points clear,” Shearer told BBC Sport.
“City’s dominance is a worry for all the top teams because of the financial clout Pep Guardiola’s side have, and the way they won the title.
“They are already a long way ahead of the rest and they are going to invest again this summer – I am pretty sure of that – so they are only going to get stronger.
“United will need to bring some new players in if they are going to get nearer to City, but they also need a new mindset – and their fans will tell you that they need a better brand of football.
“For some clubs, the way they play is very important. For example, Everton did not sack Sam Allardyce for finishing eighth, they sacked him for the style of football his team plays.
“United fans want some more flair as well. They have been used to being entertained for so long but some of the football they have seen this season has not done that. It has at times, but not often enough.”
Manchester United haven’t won the Premier League title since 2013 when Sir Alex Ferguson led the Red Devils to the top-flight crown for the 13th time.
The 20-time English champions lost 2-1 to La Liga side Sevilla in the Champions League round of 16.
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