Barcelona assistant coach Jordi Roura echoed Celtic's recent concerns over Undiano ahead of the latest 'El Clasico' which ended in a 1-1 draw in the first leg at the Bernabeu.
Undiano is already in the limelight after he was pilloried by a number of fans, former referees and former players for his controversial handling of the Champions League last-16 match first leg that saw Juventus win 3-0 at Celtic Park.
Celtic have officially asked UEFA for clarification over Undiano's performance after he failed to issue a red card or penalty against the Italian champions despite several incidents of man-handling at corner-kicks and free-kicks by Juve defenders.
But Mourinho said: "I prefer to stick to the lessons we have received in the past from Barcelona. Lessons of sportsmanship, of how to act in football.
"I do not speak about referees, nor crowd around them, nor look for yellow cards for opponents, nor conceal things. And, of course, lessons in how to play football, which they do very well. We must try, humbly, to follow that line.
"This is an isolated game, with nothing to do with past ones. Anyway, that was a long time ago. We have not played there for months.
"We must try and play as well as possible to get the best result possible. Form does not matter so much in games between the best teams. The moment in which the game is played has no significance."
Xabi Alonso and Karim Benzema are available for Real having missed the 2-1 Liga win over Deportivo la Coruna on Saturday due to injury.