Slovenia international Oblak, whose form helped Atletico ease through the group stage, had been sidelined since December after dislocating his left shoulder and having an operation.
"We will see tomorrow (Tuesday) if Jan Oblak is fit to start. We will watch him in training and decide after that," Argentine Simeone told reporters on the eve of the match.
Atletico, twice losing finalists in the last three seasons, are aiming to reach the last eight for the ninth time.
"Leverkusen are a very strong side, particularly on the wings with Karim Bellarabi and Kai Havertz," Simeone said.
"Up front they have Javier Hernandez, who is in fine form. We will see how they line up tomorrow, but they will be looking to make life as tough as possible for us."
Mexico forward Hernandez has scored four goals in his last two games while Leverkusen have not lost at home in their last 10 European matches.
"We have played many games in the Champions League in the last couple of years, but tomorrow is all that counts," said Simeone, whose side lost 1-0 at Leverkusen two years ago before advancing on penalties in the return leg at the same stage.
"We are playing against a side with great intensity, but we have great intensity too. Passion is very important in football, so I am curious to see how it will play out tomorrow," he added.
(Reporting by Karolos Grohmann; Editing by Ken Ferris)
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