Promoter Bob Arum said final negotiations were scheduled with Pacquiao adviser Michael Koncz for the fight, which would be the first for the Filipino since he lost a hotly disputed decision to Timothy Bradley on June 9.
A press conference is scheduled in Los Angeles on Tuesday to announce the fight, which will be held at the MGM Grand hotel in Las Vegas.
"It's certainly going in that direction," Arum said of Marquez being the opponent.
Marquez was one of three opponents considered by Pacquiao for the fight.
Bradley was another, but the weakness of the pay-per-view sales for his upset of Pacquiao to win the WBO Welterweight title worked against him even though there was a promoter's option for a rematch.
Miguel Cotto declined to take the fight, leaving Marquez as the likely opponent.
Pacquiao has won two of his three fights with Marquez, with the third scored a draw. But some think Marquez won them all, including last November's fight when Pacquiao got a majority decision to retain his welterweight title.