"That's what the doctor said, he said that although some of the symptoms were gone, I was not healed completely," Westbrook said Wednesday.
"Me and (head athletic trainer) Rick (Burkholder), the trainers and the coaching staff, we did every test we could, but until you get hit, you're not so sure if you're healed completely. It's not like an ankle, you can feel it every single day or a knee. A concussion is a different thing for me. It just wasn't healed completely."
The 30-year-old former All-Pro running back sustained his first career concussion when he hit his head on a defender's knee during a Monday night win at Washington on Oct. 26. He briefly lost consciousness but walked off the field under his own power.
Westbrook sat out two games and returned against San Diego on Nov. 15, only to sustain another concussion when he got sandwiched between a blocker and a tackler on a screen play.
He has since been examined twice by specialists in Pittsburgh, and has been cleared to return to practice. Westbrook will work with the scout team, and coach Andy Reid hasn't ruled him out for Sunday's game at the New York Giants.
"You gradually get him back in, take baby steps," Reid said. "I'm not going to throw him in if he's not feeling right."
Westbrook didn't sound like a guy who expects to play this week.
"I think all the signs right now are aiming toward being able to play before the end of the season," he said.
While Westbrook eases his way back, wide receiver DeSean Jackson expects to play against the Giants after missing one game with a concussion. The dynamic playmaker was a full participant at Wednesday's practice.
"I'm pretty much good," Jackson said, "back to normal, back to being DeSean Jackson."
Though he's eager to help the Eagles (8-4) pursue a Super Bowl, Westbrook is taking a cautious approach.
"I'm more concerned about how things will happen for me in the future, how having concussions now will affect me 20, 30 years from now," he said. "I don't think I'm scared to play the game of football. I'm concerned that things that are happening now, concussions, head injuries, can affect my life down the road. That really concerns me."
So why risk coming back?
"I still think I can play, still think I can produce," Westbrook said. "And I've learned from the doctors that I won't have a high risk of getting another concussion after healing completely and that's the No. 1 thing is to heal completely. I want to play football. That's what I love to do."