The Red Devils will enter the new season hoping to piece together a sustained bid for the English top-flight crown.
That cause has, however, been hindered by a lack of summer transfer activity and the delayed return of key men to the fold from World Cup duty.
It is possible that the likes of Paul Pogba and Romelu Lukaku could make an opening date with Leicester City on Friday, but Mourinho says his side cannot expect to be in peak condition.
The Portuguese told MUTV: “There are no miracles. The players with a proper pre-season, they are fit and they are strong.
“For example, Alexis has played lots of matches and lots of minutes, he is more than ready.
“But you cannot have the players that arrived on the last Monday, after World Cup semi-finals, finals and three weeks of holidays, you cannot expect them to be fit.
“Apart from that, we have some injuries. Some are expected and some are unexpected.
“[Nemanja] Matic, we weren't expecting him to be injured and needing a surgery after the World Cup. [Diogo] Dalot, we knew he had a surgery on his meniscus and we have to wait a couple of months before he is ready.”
Mourinho has also revealed the Ander Herrera, Sergio Romero, Marcos Rojo and Antonio Valencia will play no part against the Foxes, leaving him short of options.
He is, however, ready for the resumption of competitive action, with it vital that United put their summer struggles behind them and pull together in the pursuit of collective goals.
“Now it’s serious stuff,” added Mourinho.
“It’s not anymore about friendlies, it’s not anymore about preparation, it’s about points. Any point can be crucial - you don’t know what is going to happen in the end of the season.
“You have to try your best to get points and even with the difficulties that we had with our pre-season, now I don’t think is the time to be speaking about that. It’s time just to say, 'Tomorrow we have a Premier League match and we have to play to win it.'”