The Dutchman sustained a groin injury that kept him out of the 4-1 derby humiliation against Manchester City on Sunday and has not trained this week.
He was pictured out and about on Monday leaving a chemists in Alderley Edge with a box of athlete's foot cream but hasn't been at Carrington.
Speaking ahead of the match, Moyes said: 'I don't think Robin will play [against Liverpool]. He will have an outside chance of playing on Saturday.'Moyes is expected to make a number of changes for the cup tie at Old Trafford, including a first start of the season for Javier Hernandez.
'It's always a big game when you play a cup game and Liverpool are our big rivals. I always look to rotate and give young players a chance.
'Maybe this does take on more significance. Luis Suarez coming back means that Liverpool will have a really good player.'
The Mexican has made three substitute appearances so far this season as Van Persie, Wayne Rooney and Danny Welbeck have been favoured in the forward line.
Shinji Kagawa, whose only start came in the Champions League opener with Bayer Leverkusen last week, could also play.
Nani and Jonny Evans are also likely starters for the Liverpool match but it will come too soon for long-term absentee Darren Fletcher.
Liverpool will be boosted by the return of Luis Suarez, whom manager Brendan Rodgers described as 'champing at the bit' to get back on the pitch after a 10-game suspension for biting Chelsea's Branislav Ivanovic.