Madrid have not officially confirmed any new injury for Bale, but Zidane admitted at a news conference there was a problem, with the Wales global having not taken part in group training since the club's return from Germany and missing Saturday's session along with the injured Marcelo, Mateo Kovacic, Theo Hernandez and Karim Benzema.
Zinedine Zidane's side was a long way from their free-flowing best, but Isco's goals in either half were enough to end a run of three league matches without a win at home and keep pace with leader Barcelona, which remains seven points ahead of it.
Real Madrid record their first home La Liga win of the 2017-18 campaign courtesy of a 2-0 success over Espanyol on Sunday night.
Real Madrid are still dealing with numerous injuries, with Toni Kroos, Marcelo, Karim Benzema, Mateo Kovacic, and Theo Hernandez all hurt.
In his debut half-season at the helm it was a 6-0 victory against Espanyol that brought purring in the Spanish press over Zidane and his reinvention of a side that had been listless and leaderless under Rafa Benitez, who never really fit the company profile despite his sterling work at Castilla. The playmaker has become of the most important players at the Santiago Bernabeu over the past few months and that was on display this evening.
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Bale, who signed with Real Madrid in 2013 has missed 9,346 minutes since he arrived at the club.
Ronaldo was instrumental again as his surging run created the space for Isco - who started the move by winning possession - to collect Asensio's pass and slide into the bottom corner past Lopez.
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Defenders: Vallejo, Ramos, Varane, Nacho, Achraf and Tejero.
Real Madrid were struggling in the second period, but Espanyol were full of ideas, and Baptistao came close to finding Moreno free inside the home side's box in the 63rd minute as the visitors continued to threaten.
Marco Asensio has been involved in more goals than any other Real Madrid player in La Liga this season (four).