Ralf Rangnick said Cristiano Ronaldo's substitution was both tactical and rotational after the Manchester United forward returned from injury.
Ronaldo missed the FA Cup and Premier League matches against Aston Villa last week through injury but returned to the starting XI in United's 3-1 win at Brentford.
The 36-year-old played a key role in United's second goal with a deft lay-off for Bruno Fernandes to run onto and assist Mason Greenwood, but Ronaldo was surprised to be substituted for Marcus Rashford in the 71st minute.
Ronaldo appeared to ask technical director Darren Fletcher, "Why did you take me off?" Rashford made it 3-0 in the 77th minute and Rangnick, who had not explained his decision to Ronaldo, then went over and enlightened him.
"Okay, it's normal," Rangnick told BT Sport when asked about Ronaldo's reaction. "As a striker, he wants to score goals, he came back from a little injury, so it was also important to bear in mind we have another game in three days' time (against West Ham).
"On the other hand, we were 2-0 up, the same result at Villa Park and I decided to we just defend that lead this time and I think it was the right decision to switch to a back five. We scored the third goal and then in the end we wanted to keep a clean sheet.
"Unfortunately, we didn't manage to keep a clean sheet but made sure nothing happened anymore."
