The inspirational skipper went off with a groin injury just 24 minutes into Tuesday's shock Champions League defeat at home to Lyon.
It was the same problem that had kept him out for 10 days before the Lyon game. And he is racing against time to get fit for United's visit to Anfield.
Under-fire boss Rafa Benitez said: "We will continue to assess the situation and take the advice of our medical staff.
"We have more time before Sunday's game and it is too early to make any decisions one way or the other, so we will keep monitoring the situation."