Modric, 31, was given a one-game suspension after being sent off during the Spanish Super Cup played between Los Blancos and Atletico Madrid in August 2014.
The former Tottenham Hotspur maestro has been unable to serve that suspension since then as Real have not played in the competition since losing to Atletico.
Marca and AS both claims that the Spanish Federation has confirmed that Modric will need to serve the ban on Sunday when Real visit Barcelona for the first-leg of the tie - a competition contested between the champions of La Liga and the Copa del Rey from the previous campaign.
Modric will be available for the decisive second leg to be played at the Bernabeu on Wednesday [16 August].
The Croatia international has become a central figure for Real in recent seasons and the news will surely be a blow for manager Zinedine Zidane. The French boss does still has plenty of options at his disposal, including Mateo Kovacic, young Marco Asensio or new summer signing Dani Ceballos.
Furthermore, Cristiano Ronaldo may start against Barcelona after the Portugal international made a cameo appearance during the 2-1 victory over Manchester United in the the Uefa Super Cup.
Ronaldo only returned to training over the weekend following an extended break. However, should he start against Barcelona, Zidane will be able to return to his fabled 4-3-3 formation, with Isco covering for Modri'c absence in midfield.
However, Zidane hopes that the impressive win over Jose Mourinho's side will serve as a springboard ahead of the double-header with Barcelona.
"We know we've got a lot of talent and that with hard work we can achieve great things. We're very hungry, we always want more. It was a near perfect performance. The first half was fantastic and then we struggled a bit at the end of the second. If we'd have got the third goal, that would have been it. We showed great character and hunger," Zidane said in the press conference following the win over United.
"We know that every game will be more difficult and we'll come up against very good opposition throughout the season. The first comes on Sunday [against Barcelona]. We want to give it our all and we've got the ambition to do better and better. Today was no comparison with the pre-season games. We were on it from the first minute to the last".