Reeves was running forward near the halfway line during the latter stages of the 4-0 fourth-round replay reverse when Brazilian defender Luiz ran across his path and connected with his shoulder in the Brentford midfielder's jaw.
Luiz received a yellow card, which could easily have been red, while Reeves was forced off the pitch with concussion.
But the 19-year-old fortunately recovered quickly and, having received a post-match apology from Luiz, returned to action on Monday to play 90 minutes for Brentford's Under-21 side in a victory over Bristol City.
"I feel fine," Reeves told the club's official website. "I was alright about 10 minutes after and was sitting on the bench.
"I remember passing it off to Clayton Donaldson and a big shoulder smashed me straight in the face, that's it.
"The medical team just took me off, there was no talking. You could tell I wasn't happy to come off. I was pushing the physios off, which wasn't very clever, but I didn't know what I was doing at the time."
- Sports & Recreation