Atletico and Lyon have been in "very advanced" negotiations regarding a deal that will see Dembele move to the Wanda Metropolitano on loan with an option to buy.
Atleti will reportedly be able to secure Dembele for a €35million fee should the former France Under-21 international impress on loan before the end of the season.
Dembele has scored just once in 16 Ligue 1 games for Lyon this season, having hit 24 in 46 across all competitions in 2019-20.
Simeone was asked about Dembele following Tuesday's 2-0 LaLiga victory over Sevilla in the Spanish capital.
"It is very advanced," Atletico head coach Simeone said post-match. "We are going to confirm it."