Portland Trail Blazers center Robert Williams III has spent a frustrating few weeks working through concussion protocol.

He was only able to train on Thursday and was unable to take part in any games. He watched from home as the team lost five of six games without him since he went down in Memphis on Nov. 25.

“It was scary, but injuries are part of the game,” Williams said.

Scary because Williams, whose game against San Antonio on Friday is questionable, felt lightheaded, nauseous and suffered from a headache.

Williams suffered numerous injuries to his body. A knee injury cost him 76 games last season. He started this season with a hamstring injury.

These injuries improve as a player gains mobility and strength. Concussions require players to go through a series of tests to return to the field.

“It’s been a long road back,” he said.

Blazers coach Chauncey Billups said he was confident Williams would play this weekend.

“He looked good,” Billups said. “He looked like Rob. But he’s still in the protocol, so he still has a level or something he has to pass. So we’ll see if he can pass today or tomorrow.”

Before each season, players undergo a basic test of their brain function. After a concussion, you will go through the same tests and will not be cleared until you reach your previous season’s performance.

Williams remained just a few steps away from being cleared to play in games.

“I have boxes to check off,” he said.

Thursday’s training was a positive step.

“Just grateful to be back out here,” he said.

The NBA adopted its concussion protocol in 2011.

Billups, who played from 1997 to 2014, remembers when the protocol didn’t exist.

“That might be why I’m so confused,” Billups said jokingly.

Back then, the players suffered all sorts of injuries. Billups said he simultaneously broke his right wrist and nose while playing in Denver and fell to the ground after being undercut on a fast break.

“I played the rest of the game and never missed a game,” he said.