The Portuguese coach stated that he wanted to test new things during the match and insisted he is only interested in winning official contests
The Spanish capital club fielded a much-changed team at the Estadio da Luz and were surprised by a rampant second-half display from the hosts after going into the break with the scoreline at 2-2.
After the match, Mourinho admitted he did not take a serious approach to the game and warned Benfica not to get carried away with the result.
"There is no such thing as heavy defeats in friendly matches," he told reporters. "We knew the risk we were taking. Benfica did not wish to test new things, they played seriously.
"I hope this victory will boost their morale, but in a balanced way. Because if they give too much importance to this result, it could be negative for them.
"I hope to return here during the season, but to play an official match, and to win when winning matters."
Meanwhile, Fabio Coentrao admitted that Benfica were worthy winners and shared the emotion to return to the Estadio da Luz one year after leaving the Portuguese club.
"It was a good game and a good spectacle," Coentrao added. "We did not expect this result, but we were missing many players and we had several others from the Castilla team. But Benfica deserved to win.
"I am a Benfica fan and I did not want to waste the opportunity to play in my stadium once again."