Heyward-Bey was released from the hospital on Monday, a day after being knocked out by Mundy's helmet-to-helmet hit, and is expected to make a full recovery.
Heyward-Bey suffered a concussion and a strained neck on the scary hit that stopped the game Sunday for more than 10 minutes as Heyward-Bey had to regain consciousness before being carted off the field and taken to the hospital.
There was no penalty called on the play by the replacement officials.
On Monday, coach Dennis Allen said Heyward-Bey sounded tired when he talked with him and there is no word on when he will be cleared to return to the field.
"The concussion is obviously the bigger issue than anything else right now," Allen said. "He's a guy that we're going to have to continue to evaluate and see where he's at. We were all pleased to see that it wasn't anything severe as far as neck injury or anything like that."
The injury happened early in the fourth quarter of Oakland's 34-31 victory. Heyward-Bey was running across the end zone to catch a pass from Carson Palmer when Mundy launched his body and lowered his helmet into Heyward-Bey's facemask.
Heyward-Bey's neck jerked violently and his head also crashed into the ground as he lost consciousness. The pass was incomplete.
Mundy said after the game that he did not intentionally lead with his helmet and felt bad that another player was injured.
Allen said he did not believe there was any intent to injure on Mundy's part.
"It's the game of football," Allen said. "I don't think people are trying to go out there and hurt people. The safety was playing the game fast and physical. It's a tough game to play when you're making split-second decisions on how you play the game. It's the unfortunate things that happen in this game but we move on from it."
Information from The Associated Press was used in this report.