The Red Devils have struggled to produce consistent form in the top flight this season and they were also dumped out of the Champions League at the last 16 stage this term.
Manchester United are currently in second place in the Premier League table as they look to finish as close to Manchester City as possible.
Jose Mourinho’s men slumped to a disappointing 1-0 loss to West Bromwich Albion in the Premier League at Old Trafford last weekend to hand Manchester City the title.
The Red Devils are now only playing for a second-placed finish and the FA Cup, with Mourinho’s side set to take on Tottenham Hotspur in the semi-finals at Wembley next weekend.
And former Arsenal star Merson feels that Manchester United will have to bolster their squad with quality this summer and also improve their tactics if they want to win the title next term.
Speaking to Sky Sports, Merson said: “Manchester United need to improve, both tactically and with personnel. Mourinho is a serial winner, don’t get me wrong, he’s a phenomenal manager, but the game is changing.
“You need to win more games. We saw in Europe, getting a goalless draw at Sevilla, you can’t do that in this day and age. You have to get a goal away from home! Juventus went to Real and got three, Liverpool went to Man City and got two.
“There is too much attacking flair on the pitch, and defenders are nowhere near as good as they used to be.”
Manchester United will take on Tottenham Hotspur on Saturday at Wembley for a place in the FA Cup final against either Southampton or Chelsea FC.
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news