Goals from Anthony Elanga, Mason Greenwood and Marcus Rashford boosted Manchester United’s top-four ambitions.
But the spotlight was once again on Ronaldo, who was far from impressed when his number came up with 20 minutes left.
The striker trudged off slowly before hurling his coat to the floor as he sat down muttering ‘why?’.
However, United interim boss Rangnick said: “He was asking ‘why me’?. But I told him I take the decisions in the interest of the team.
“We were 2-0 up at Aston Villa last week with 15 minutes to play and we didn’t want to make the same mistake, so I decided to switch to a back five.
“Cristiano was not happy, he is a goalscorer. But for us it was more important to be compact and defend their set-pieces.
“I said to him I know you are still ambitious and you want to score, but maybe in a couple of years when you are a head coach you will make me the same decision.
“I didn’t expect him to hug me after being substituted but I take my decisions in the interest of the team and the club. I have never had any issue with Cristiano.”
Teenage winger Elanga, starting only his fourth Premier League match, broke the deadlock early in the second half after Brentford had spurned a host of chances to take the lead.
England forward Greenwood then notched his first Premier League goal since October and Rashford climbed off the bench to add the third and secure United’s first win of 2022.
It was a bitter blow for Brentford, who had bossed the first league meeting between these sides since 1946 – Mathias Jensen alone could have had a first-half hat-trick were it not for David De Gea’s legs – but only had Ivan Toney’s late consolation to show for it.
“I think we have to be honest, without David and his brilliant saves it would have been difficult to be level at half-time,” added Rangnick, “The first half was not good.
“We have to admit there was an opposition on the pitch who were aggressive, brave and pressed us high. The second half we did a lot better.
“In the end we could have scored four or five, but we couldn’t have complained if we had been 2-0 down at the end of the first half.”
Brentford have now lost five of their last six matches but boss Thomas Frank could not fault his side.
“I know goals change the momentum of games but if you look at the overall performance over 90 minutes there was only one winner, and that was us,” he said.
“I’m unbelievably proud of the team and the players and the way we absolutely destroyed Manchester United in the first half. We had six chances. They had zero
“It’s a miracle they were not 2-0 down at half-time. My boys always keep running and fighting and they did that until the end.”