The France international has been struggling with a groin problem and was left out of Manuel Pellegrini's squad on Tuesday along with the injured Royston Drenthe and Pepe, and suspended Marcelo and Xabi Alonso.
Real need to overturn a 1-0 first leg deficit, and to beat a side who have drawn on their only two previous visits to the Bernabeu in this competition.
The absence of former Liverpool midfielder Xabi Alonso, one of Real's most consistent performers of late, does not help Pellegrini resolve his toughest decision, which is who to play in the middle.
The attack-minded Guti and Rafael van der Vaart came on as second-half substitutes and helped inspire Real to their 3-2 comeback win against Sevilla on Saturday, and would be popular choices on Wednesday, but the Chilean gave little away.
"The last 20 minutes of the first half against Sevilla were very good as well (as the second half)," he told a news conference.
"That midfield has played together eight or nine times and won praise for its performances against Barcelona and Valencia among others. I'm not affected by criticism.
"The atmosphere at the Bernabeu will be special and we need to try and make the most of this. It is a narrow deficit we face and I believe we can overturn it to make the quarter-finals."
