Mourinho's arrival in Manchester was supposed to herald an upturn in United's fortunes after they failed, under his mentor Louis van Gaal, to qualify for the Champions League for the second time in three years last season.
But United are currently six points off a top four position in the Premier League needed to secure a Champions League spot - despite a pre-season spending spree that included splashing out a world record £89 million transfer fee for midfielder Paul Pogba.
There are now suggestion that Mourinho, once the self-styled 'Special One', may be losing his touch amid reports that senior players are unhappy with the Portuguese's methods.
But Herrera insisted that Mourinho still had the trust of United's first-team squad.
"We have a saying in Spain: you catch the liar before you catch the lame man," Herrera told the Daily Mail. "It's true in life and even more so in football. If a coach is not straight, players soon find him out. Mourinho is very straight.
"He says what he thinks and he says it to your face. He says it when it is something we will like, but he also says it when it is something we won't like.
"The team is enjoying the day to day. Training is very dynamic and everything is with the ball so the players never lose concentration - there are no really long sessions or long team talks.
"He really understands what a footballer needs which is 20 minutes of team talk and then you get down to it."
Herrera, 27, could make his senior Spain debut against England in a Wembley friendly on Tuesday after becoming a United regular under Mourinho having been given a deeper role by his club manager.
"I have come here because Mourinho has given me the opportunity to take on a new position and I feel comfortable with it," Herrera said.
"I'm more defensive now, playing in front of the back four. I think I have that ability to win the ball back quickly when we lose it.
"I have taken a leap forward at United which has allowed me to get into the national side."
Original source: Herrera adamant Mourinho has United backing