The former Real Madrid and Chelsea manager will be eager to bounce back from his second straight loss this season when United travel to Watford.
"It is obvious that we left players like (Antonio) Valencia and (Luke) Shaw at home to play and to rest so we go back to a normal team without so many changes," Mourinho told BT Sport.
Mourinho handed starts to forwards Marcus Rashford and Anthony Martial, while record signing Paul Pogba retained a spot in the line-up against the Dutch side, but a laboured offensive display from United lacked any fluency.
"Look I don't want to personalise. Some of the players were playing their first minutes of the season, not many minutes for them," Mourinho added.
"Obviously some played better than others but I have to understand that some of these players haven't played week after week like the other boys.
"The second half was better than the first. We had more intensity than the first half because in the first my players were in control without pushing to try but the second was different.
"Feyenoord defended with everything, a great crowd, a great spirit we know that."
United, who suffered their first league defeat of the season against Manchester City last Saturday, are fourth in the table.
(Reporting by Nivedita Shankar in Bengaluru; Editing by Ian Ransom)
