Pacquiao next faces Jeff Horn on July 1, though his schedule appears to be open after that fight.
"If McGregor is still looking for an opponent, Manny Pacquiao is there," Arum said.
He also offered some advice to UFC President Dana White, who's negotiating the potential McGregor-Mayweather bout.
"Be patient. Don't give in," Arum said. "Dana's a good negotiator, and he'll get the thing done, but he can't be pushed around by Mayweather and his people."
The two camps have continued their prolonged discussion, and White put the proverbial ball in Mayweather's court May 17.
"The McGregor side is done," White said on TNT after Game 1 of the NBA's Eastern Conference Finals, per ESPN.com. "I'm starting to work on the Mayweather side now.
"I'm not saying the fight will happen, but I've got one side done. Now it's time to work on the other. If we can come to a deal with Haymon and Mayweather, the fight's gonna happen."
Said McGregor in a statement to TheMacLife.com the next day: "The first and most important part of this historic contract has now officially been signed off on. Congratulations to all parties involved. We now await [Mayweather adviser] Al Haymon and his boxer's signature in the coming days."
Mayweather has said he would meet with Haymon and Mayweather Promotions CEO Leonard Ellerbe to work out the details of the McGregor fight, per Scott Gleeson of USA Today. But if he doesn't, Pacquiao appears to be waiting in the wings.