"There's a lot of unknowns about brain injury and the brain itself," Goodell said. "We're all learning."
Other items addressed by Goodell on Friday:
— The NFL would like to start testing for HGH this offseason; the league and players haven't been able to resolve an impasse even though the new, 10-year collective bargaining agreement added that performance-enhancer to the drug program. On Thursday, NFL Players Association executive director DeMaurice Smith said: "No one will bully us into a test."
"We have been working to try to address the issues the union has raised," Goodell said. "We believe the science is clear. I do not hear any dispute from scientists around the world on the fact that this test is valid and we have the basis to put in and implement an HGH test that is fair to the players. We expect to be able to do that."
— Asked about complaints from the city of St. Louis about last month's announcement that the Rams will play one "home" game in London in each of the next three seasons, Goodell said: "We're going to play the London games. We hope it will be with the Rams and the New England Patriots next year. That's what we planned. ... But there are issues that obviously are going to have to get resolved. We know there are discussions going on. We hope that will get resolved shortly."
The league is looking into bringing a regular-season game back to Mexico, and would like to eventually consider other sites in Latin America. Similarly, if its current plans for games in London prove successful, other spots in Europe could get games.
— All the buzz and speculation this week about injured Colts quarterback Peyton Manning, who didn't play this season, doesn't bother Goodell, who said that hasn't been a distraction in the run-up to Sunday's game between the New York Giants and New England Patriots.
"I understand the interest," Goodell said.
As for whether Indianapolis might host another Super Bowl, Goodell said that's a possibility, because "they're doing a fantastic job."
AP National Writer Nancy Armour and AP Sports Writer Joe Kay contributed to this report.
Howard Fendrich can be reached at http://twitter.com/HowardFendrich
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