Bradley (28-0 with 12 knockouts) worries the renowned trainer because he is in his prime and brings in dirty tactics to the ring.
The Filipino eight-division world champion battles Bradley on June 9 in Las Vegas.
“He’s a dirty fighter, well, he’s a physical fighter,” Roach told Chris Robinson of examiner.com Saturday. “He uses his head, elbows, so forth.
“Manny does have some trouble with that because Manny can’t retaliate. I tell Manny that if a guy hits you low, hit him back low, but he won’t do that. It could pose a bit of a problem.”
The choice of Bradley was formally announced by Michael Koncz, Pacquiao’s adviser, on Monday (Tuesday in Manila).
“Bradley is a young, undefeated fighter who deserves a chance,” Koncz told ESPN.com’s Dan Rafael.
Koncz added that the 28-year-old Bradley’s selection over two other contenders—Lamont Peterson and Juan Manuel Marquez—sits well with Pacquiao, who will be staking his World Boxing Organization welterweight crown against his light welterweight counterpart.
“Manny is excited,” said Roach. “Until we name an opponent, boxing is out of Manny’s mind. He’s busy working in (the Philippine) congress. But he gets interested in boxing again when he has an opponent and a fight.”
Though Bradley holds court in the 140-pound division, he’s fought at 147 lb and 152 lb.
Bradley’s sterling record includes a 12-round unanimous decision over Lamont Peterson in 2009 and Devon Alexander last year.
Pacquiao is expected to receive a guaranteed $20 million against Bradley.