Madrid started 2013 with a league victory over Real Sociedad on Sunday, but it was not the smooth return to action the reigning champions would have liked.
Coach Jose Mourinho was subjected to widespread jeering prior to kick-off after again selecting Antonio Adan in place of long-serving captain Iker Casillas in goal, and then six minutes into the match Adan was sent off after conceding a penalty.
The 10 men of Madrid still managed to win, but the hard-fought 4-3 victory failed to close the 16-point gap to Primera Division pacesetters Barcelona and was hardly the ideal preparation for an important cup tie in three days' time either.
Mourinho said he accepted the supporters' ire due to the team's disappointing league form, but expects the fans to be up for the cup on Wednesday where Madrid will try to overturn a 2-1 deficit against Celta.
He said: "Celta play well, are confident, are leading 2-1, but my experience in these two and a half years is that the fans like the cup, the excitement of winning and going through and I have a feeling that even though we are playing again inside three days, if we play with the same effort (as against Sociedad), the team and the fans will be ready and we can go through."
Holders and 26-time champions Barcelona will be confident of reaching the last eight as they hold a 2-0 lead over second division Cordoba heading into Thursday's return meeting at the Nou Camp.
Barca, who could meet Madrid in the semi-finals, made it 25 wins in 29 competitive matches this season with a 4-0 derby mauling of Espanyol on Sunday in a match which saw coach Tito Vilanova return to the bench less than three weeks after undergoing throat surgery.
Sevilla, Atletico Madrid and Valencia also look good bets to progress through their last-16 ties as they each hold healthy leads against Primera Division opposition.
Sevilla, the 2010 winners, boast a 5-0 lead and home advantage for the second leg against Real Mallorca, while Atletico make the short trip to local rivals Getafe having won the first leg 3-0.
Valencia, meanwhile, defend a 2-0 lead when they play host to Osasuna.
In other matches, third tier Eibar and second division Las Palmas are level at 1-1 heading into their respective second legs at Malaga and Real Betis, while Real Zaragoza host Levante holding a 1-0 lead.