Both players were involved in the recent international break, with Coutinho starting both of Brazil's matches including an 87-minute stint during the 3-0 win over Chile in the early hours of Wednesday morning.
Wijnaldum, meanwhile, played 71 minutes for Netherlands on Tuesday evening as they fell short of World Cup qualification despite a 2-0 win over Sweden.
The Liverpool Echo reports that both players took part in a recovery session inside while the rest of Jurgen Klopp's squad trained on the outside pitches at Melwood.
The Reds will already be without Sadio Mane for Saturday's crunch match against their fierce rivals, while Coutinho and Wijnaldum are now considered doubts for the Anfield showdown.
Liverpool are already seven points adrift of United and could slip 10 points behind after just eight games with a defeat.