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.
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."
Atletico extended their lead atop LaLiga to four points thanks to goals from Angel Correa and Saul Niguez against visiting Sevilla.
Correa broke the deadlock in the 17th minute before Atletico team-mate Saul put the result beyond doubt with 14 minutes remaining.
Of Correa's goals for Atletico, 19 out of 39 across all competitions have been the opening goal of a match, per Opta.
Atletico, meanwhile, are one of the two teams with the most clean sheets in the top five European leagues this season (11 in 16 fixtures) – level with Ligue 1 champions Paris Saint-Germain.
On Saul, who has only started seven of 13 LaLiga games this term, Simeone said: "Like everything in life, the first one who has to want to get out of the difficulty is oneself and Saul loves it.
"He knew he was not well and has always worked. Since he was 18 years old, he has had to live things very good and now he has to live another and Saul is helping himself and is already getting out of the situation he was in."
Simeone was named the best coach of the decade by the International Federation of Football History and Statistics (IFFHS) this week.
"I am not aware of that," said Simeone, who guided Atletico to LaLiga glory in 2014 and Champions League finals in 2014 and 2016.
"We focus on improving the team, we work daily, we have made an important change in the game. That is healthy and gives us great joy for the number of players who have joined us and that allows us to be.
"With the energy of the first day I got on the plane with the mentality of taking the team to the top of Europe."