Marcelo and Modric pick up muscle injuries
Brazil left-back Marcelo fell awkwardly during a challenge after only 12 minutes at the Bernabeu, and was carried out of the stadium on a stretcher.
Croatia midfielder Modric was substituted midway through the second half after pulling up holding the back of his thigh.
"The injuries were the worst aspect of the game," Real assistant coach Aitor Karanka told the post-match news conference. "Marcelo felt a sharp pain. Modric's problem seems only to be a strain, but he said he felt something and we'll do some checks."
Goalkeeper Diego Lopez and striker Karim Benzema also seemed to pick up knocks during the encounter.
Lopez was unable to take the goal-kicks near the end, and was holding his hip, while Benzema was replaced by Gonzalo Higuain after having complained of a kick to his ankle, though Karanka said neither injury seemed particularly serious.
Real fielded a weakened team against Betis and have made it clear their priority for the remainder of the season is the pursuit of tenth European Cup triumph, rather than trying to overhaul runaway La Liga leaders Barcelona.
Jose Mourinho's side travel to play Borussia Dortmund on Wednesday.