The Teessiders were 1-0 down when Crooks was dismissed for a collision with Grant Hall, a clash which forced the Boro man to be substituted.
Rotherham boss Paul Warne wasn’t happy with the decision, yet Warnock believes the officials got it right.
“Luckily he’s not concussed, thank goodness,” Warnock told Sky Sports when asked about Hall. “He’s just cut his head. He’s just having stitches now in his head.
“I thought it was a red card. I thought he endangered the player with the challenge so it’s how you see it I suppose.”
The result left Warnock’s former side Rotherham four points from safety, although they do have games in hand on Derby above them.
Warnock decided to bring on 21-year-old Connor Malley in place of Hall after naming an experienced substitutes bench, which included just five outfield players with no Championship experience.
Malley impressed on his senior debut for Boro, and Warnock felt he would be better suited to the game than fellow midfielder Hayden Hackney.
Goals from George Saville and Chuba Akpom gave the visitors the points, while Jonny Howson hit the crossbar on two occasions when the score was 1-1.
“We played with a bit of a freedom with nothing at stake and passed it around well,” added Warnock.
“We had numerous chances, Howson was aiming for the crossbar.
“We're glad to get the result, but just a shame it's Rotherham. I want them to stay up.
“Connor did well. They all have. We picked Connor over Hayden because with the man advantage Connor is a bit better with the ball.”
Boro could have another big say in the Championship relegation battle when they host Sheffield Wednesday this weekend.
Wednesday are four points from safety with four games to play and, as a former Sheffield United boss, Warnock says he’s relishing the contest.
“I’m absolutely looking forward to it,” said Warnock.
“I wish the fans were in because I always get a nice welcome from the Wednesdayites.”