The Old Trafford manager spent a combined £68 million ($87m) on Brazilian midfielder Fred and young Portuguese defender Diogo Dalot in the last transfer window.
Neither have made a huge impact at Old Trafford with Fred failing to win a regular place in United's midfield and Dalot seen as a long-term replacement for Antonio Valencia.
Mourinho wanted to add an experienced centre-back, targeting Tottenham's Toby Alderweireld and also being heavily linked with Leicester's World Cup star Harry Maguire.
He made his displeasure at United's inability to boost his defensive ranks clear earlier in the season with repeated reference to it.
And he returned to the theme ahead of United's clash with Arsenal on Wednesday night with the Red Devils eight points behind their London rivals and well of the title race pace.
With United's top-six rivals, bar Tottenham, making signings, Mourinho bemoaned his team's situation - and claimed their achievements last season may have raised expectations for this season too high.
"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," said Mourinho.
"So, every team got better and we didn't.
"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."
In contrast, during the summer Arsenal signed Lucas Torreira (Sampdoria), Bernd Leno (Bayer Leverkusen), Sokratis (Borussia Dortmund), Stephan Lichtsteiner (Juventus) and Matteo Guendouzi (FC Lorient) for a total of £71m ($90m).