The Red Devils are already a substantial distance behind Premier League leaders Manchester City in the title race.
Even a top-four finish looks unlikely following four defeats and four stalemates in their opening 14 games in the Premier League season.
Manchester United conceded their 23rd goal of the season in last weekend’s 2-2 draw at relegation candidates Southampton ahead of the visit of Arsenal on Wednesday night.
Mourinho’s side finished in second place in the Premier League table last term after the Red Devils ended up 19 points behind Manchester City.
However, the Red Devils failed to bolster their defence in the summer and Mourinho has struggled to keep his defensive players fit.
Manchester United signed Shakhtar Donetsk midfielder Fred, FC Porto defender Diogo Dalot and Stoke goalkeeper Lee Grant.
But the trio have been unable to help close the gap between Manchester United and their derby rivals.
In fact, Mourinho’s will start their clash against Arsenal a staggering 19 points adrift of Manchester City.
Speaking at his pre-match media conference ahead of the visit of the Gunners, Mourinho said:
“The distance [between United and top four] is something I couldn’t predict.
“You always think in a positive way, you always think the distance is not going to be so big.
“But I said clearly, last season we didn’t get the credit we deserved. To finish second, to play the FA Cup final and to qualify as winners in our Champions League group – we didn’t get the credit we deserved at all.
“All the teams they got better, Spurs was the team that did not make a direct investment, but the best investment is to keep the top players that you have. So every team got better and we didn’t.”
Manchester United haven’t won the Premier League title since 2013 when Sir Alex Ferguson was still in charge of the Red Devils side.
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