The German manager confirmed that the Brazilian went into the Bees' dressing room after the game to say sorry for the challenge which left Jake Reeves with a concussion
Rafa Benitez's team were ahead by four goals and in complete control of the game when the Brazilian charged into Reeves with his shoulder after the ball had been played.
Rosler confirmed that Reeves was not seriously hurt and, while admitting that he had not seen the foul clearly, praised Luiz's "fantastic gesture" after he visited the Brentford dressing room.
"Jake is fine," he told reporters. "He wanted to continue but our medical staff said it’s better not to. Safety first, but he will be OK next week. He was definitely concussed.
"I haven't seen the Luiz challenge clearly enough to make a judgment. He came into our dressing room afterwards to speak to Jake, and I think that's a fantastic gesture."
Chelsea will face Middlesbrough in the fifth round and will learn their potential quarter-final opponent when the draw is made on Sunday evening.