The United boss warned supporters they were set for a tough campaign at the start of the season after the club failed to bring in his desired centre-back targets to build on their second-place finish last term.
Mourinho spent £80m on the signings of Fred and Diogo Dalot, meanwhile, the rest of the top six, except for Tottenham, were all active in the transfer market.
Despite being surprised by the eight-point gap between United and the top four, Mourinho believes last season may have distorted the perception of his side’s title credentials.
“The distance [between United and top four] is something I couldn’t predict,” said Mourinho. “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.”
Injuries to Chris Smalling and Eric Bailly forced Mourinho to field a makeshift back-three in Saturday’s 2-2 draw at Southampton, with midfielders Scott McTominay and Nemanja Matic playing alongside Phil Jones.
Asked about the injuries in his squad ahead of Wednesday’s visit of Arsenal in the league, Mourinho said the situation had worsened, but revealed one defender is ready to return.
“Against Southampton, three hours before the game I don’t know the team that is going to play,” he added.
“We were waiting and waiting to see who could [play] and who could not. And we are in a very similar situation this time.
“If the game was at 11 o’clock [in the morning on Tuesday], I would say Chris Smalling is not playing, Eric Bailly is not playing, Phil Jones is not playing, Victor Lindelof is out of the question.
“Antonio Valencia is not playing, Luke Shaw is a doubt, Matteo Darmian is back, but I have to wait, I have to select probably everybody again and take everybody again.
“Maybe [on Wednesday] we have to decide, like in the last match, with a couple of hours before the game.” skysports