The young midfielder was absent from Thursday's session, with the club confirming he was struggling with "muscle discomfort". His potential absence comes at a key moment in the title race, with Madrid chasing leaders FC Barcelona and needing to close a nine-point gap with just 18 points still available.
Head coach Alvaro Arbeloa is already dealing with a stretched squad. Rodrygo Goes and Thibaut Courtois remain ruled out, while Eder Militao and Raul Asencio are also doubts.
Asencio has only just returned to light training after illness, while concern remains around Militao, who was forced off during the recent win over Deportivo Alaves.
Arbeloa offered a cautious update on the defender's condition, saying: "Militao is undergoing an MRI now, and it seems it will just be a scare. But we don't know the full extent yet. We are remaining calm."
Madrid completed their final preparations on Thursday, but uncertainty around Guler's fitness has added to the tension ahead of a must-win fixture. With little margin for error left in the title race, Arbeloa may be forced into further changes as his side look to keep their hopes alive.