Las Vegas - Veteran promoter Bob Arum fears Filipino star Manny Pacquiao might be risking brain damage when he steps into the ring against WBA welterweight champion Keith Thurman in Las Vegas next month.

Pacquiao, who turns 41 in December, will be facing one of his most dangerous opponents in years in the shape of Thurman, who is unbeaten in 30 fights and 10 years younger than the Filipino.

The 87-year-old Arum, a legendary figure in boxing who promoted Muhammad Ali during the 1960s and 1970s, no longer works with Pacquiao after overseeing most of the eight-division world champion's signature fights.

However Arum believes the serious brain injury suffered by 41-year-old Zab Judah last Friday should serve as a cautionary tale.

Speaking on the sidelines of this Saturday's heavyweight fight between Tyson Fury and Tom Schwarz in Las Vegas, Arum admitted he feared for Pacquiao.

"Look, I love Manny Pacquiao," Arum told reporters.

"I have a whole history with Manny Pacquiao. I'm really rooting for Manny Pacquiao, but you've got to realise that he's 41 years of age, and when a fighter has been around so long passes his late 30s and goes into his 40s, he's not going to be as good as he was in his prime.

"I wish him the best and I hope he wins the fight but I am concerned, as I would be for any fighter, that when they get to a certain age that they probably should not be fighting anymore.

"You know, I mean, the doctors will tell you that the cranium as you get older, thins out. So a guy that's younger gets hit and the cranium absorbs the blow so that it doesn't affect the brain matter.

"When they get older the cranium is thinner, and when you get hit it affects - that would be the worst thing in the world if Manny Pacquiao suffered brain damage at this point."

Arum cited the case of Judah, who suffered a bleed on the brain after his 11th round stoppage by Cletus Seldin in Syracuse last week. Judah was released from hospital on Monday.

"Zab Judah is a perfect example of how dangerous it is for a guy to continue fighting into his 40s," Arum said.

Pacquiao faces Thurman in Las Vegas on July 20.