The influential midfielder, who typically plays in a central holding role for the Spanish champions, has missed Atletico's last two matches, a 5-0 Champions League success at home to Malmo and last weekend's 1-0 win at Getafe.
"Gabi is our heart," Simeone told a news conference previewing the match at the Calderon.
"There are players that mark out the rhythm of the team. When he is at his best, he helps the team play with the intensity he himself transmits."
Atletico, who play at Malmo in the Champions League Group A on Tuesday, are fifth in La Liga after nine matches, level on 20 points with fourth-placed Valencia.
Barcelona and Sevilla lead on 22 points, with Real Madrid third on 21.
Simeone said Atletico still had room for improvement and a host of new players, including Croatia forward Mario Mandzukic, France forward Antoine Griezmann and Italy winger Alessio Cerci, were taking time to settle.
After they won La Liga for the first time in 18 years and reached the final of Europe's elite club competition, Atletico sold top scorer Diego Costa and fullback Filipe Luis to Chelsea, while goalkeeper Thibaut Courtois returned to the London club after a loan spell.
"We are not at the level we can be," Simeone said.
"There is a lot to improve and I think that is normal. New players have come in and the adaptation process takes a while."
(Reporting by Iain Rogers, editing by Ian Chadband)
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