Manny Pacquiao has chosen his sparring partners in preparation for his big rematch with Timothy Bradley. Reuters

Manny Pacquiao is stepping up his training for the epic Timothy Bradley fight by choosing to spar with two seriously difficult boxers.

The "Pacman" has chosen former IBF Junior lightweight champion Steve Forbes and former WBO junior welterweight champion Kendall Holt to help hone his skills ahead of his rematch with Bradley on April 12 at the MGM Grand Garden Arena in Las Vegas.

"Pacquiao will be employing the services of former IBF junior lightweight champion Steve Forbes and former WBO junior welterweight champion Kendall Holt as sparring partners this training camp," trainer Freddie Roach told Rappler.

Another possible sparring partner for Pacquiao is Miguel Cotto, who has expressed his desire to help out his good friend as much as he can.

Both fighters train under Roach, and Cotto could also use the preparation with Pacquiao as good practise ahead of his June fight with Sergio Martinez.

Pacquiao is expected to shift his training base to Los Angeles next month, although the actual dates have not been confirmed as yet,

Meanwhile, Top-Rank promoter Bob Arum has acknowledged Pacquiao could struggle to pick up a knockout victory over Bradley, something the Filipino great has not managed to do in his last several fights.

"As we age, we're not the same as we were when we were a little younger, so that's clearly the case for everybody," Arum told Yahoo Sports.

"Manny Pacquiao, I think showed in the Rios fight that he knew how to go back to his roots. People looked at Pacquiao as being a knockout puncher because they remember the fight with Ricky Hatton, where he laid Hatton out."

Arum, though, also said Pacquiao has never been someone who has relied on power, with the boxer having plenty of other attributes in his locker.

"The truth is that Manny Pacquiao's strength was never as a knockout puncher," Arum added. "He was the kind of guy, and is the kind of guy, that is very elusive and comes from all angles, that has a southpaw style that scores punches.

"And he's so quick and when you counter him, he disappears. I mean, look at the Oscar de la Hoya fight. He never had Oscar in serious trouble of getting knocked out, but he just beat him [easily]."