Cristiano Ronaldo was a notable absentee in Manchester United's 1-4 thrashing at the hands of Manchester City on Sunday.
The Portuguese striker was missing from the traveling squad, as Rangnick said that the player has not yet recovered from his hip flexor injury.
With pundits like Roy Keane questioning the situation, Rangnick has defended his medical team and their authenticity.
Replying to Roy Keane's remarks about 'something not adding up', Rangnick said (reported via Sportsmole):
"I have to believe in my medical department. My doctor came to see me on Friday morning before training and told me Cristiano Ronaldo cannot train because of some problems with his hip flexor and the same was true on Saturday and that's why he couldn't be part of the squad."
The Manchester United boss added:
"It's just a fact, if players tell the doctor and the medical department that they are injured and cannot play. I have to accept it as a manager. I cannot force a player to play if he thinks he's not available because he has an injury."
Cristiano Ronaldo has scored 15 goals in 30 games for the Red Devils this season. However, his goals have dried up of late, with the 37-year-old scoring only once in the last 10 games for the club.
Edinson Cavani was another notable absentee
Manchester United were left short of options up front. With both Anthony Martial (on loan) and Mason Greenwood (undergoing a police case for sexual assault) out of the club right now, United had to rely on players available.
With Ronaldo also injured, it left Edinson Cavani and Marcus Rashford as the only two recognized strikers at the club. However, Rashford's poor form and Cavani's continued absence saw Rangnick start with Anthony Elanga up front against Manchester City.
Cavani has also recovered from his injury but is not fully match fit yet, as was confirmed by Rangnick.
Manchester City thrash Manchester United 4-1
City did their first double over United since the 2018-19 season. The Red Devils started the game with passion and despite going 1-0 down, managed to level the score through Jadon Sancho.
However, United could not keep up with City's precision play and ran out of steam quite early. Pep Guardiola's men then proceeded to put their local rivals to the sword by winning the match 4-1.