The former Atalanta striker struck in Atletico's 5-0 Copa del Rey quarterfinal win at Celta Vigo.
Simeone said afterwards: "His qualities speak for themselves. He's different from everyone else we have.
"He's a specialist in one-on-one situations. He allows us to play in different positions. He's had a very positive impact on the team's play.
"Hopefully, we can help him become an even better player, and he'll help us as a team."
Obed Vargas and Rodrigo Mendoza came off the bench to also made their debuts after arriving in the final days of the winter market.
On Vargas, Simeone said: "He's a young guy, like Rodrigo, eager to help the team. They inspire me; they have energy, youth, and talent. In football, age doesn't matter, there are good players and not-so-good players."
And on Mendoza, he added: “Rodrigo came to play there in the middle and he'll surely help us. He's just starting out, we told him that age doesn't matter here, players with talent and the work ethic can play whether they're 35 or 20. It's normal that Mateu is happy, like everyone else.”