Last week saw representatives of the Liga club meet with the player's agents, resulting in a preliminary agreement being struck between the parties.
Atleti must now negotiate a suitable deal with his club, something which proves to be fraught with risk given the interest that Telles has provoked in some of Europe's biggest sides.
Telles, 26, is under contract in Portugal until June 2021 and holds a release clause of €40 million (£35.2m/$45.4m) should he wish to leave the current leaders of the Liga NOS.
Porto are reluctant to sell, but the club are in far from rude financial health and the defender's exit would go a long way to balancing the books.
The Dragao remain under supervision from UEFA's Financial Control Committee, and have committed to progressively reduce their spending and budget deficit. Atleti hope to push forward with a deal that would leave Porto satisfied but which stops short of the €40m buyout.
Diego Simeone's men are acutely aware of the lack of depth possessed out wide at the back, an issue that could worsen over the coming months.
Brazilian Filipe Luis is out of contract at the Wanda Metropolitana at the end of the current season and looks unlikely to renew, while Lucas Hernandez has been consistently stalked by Bayern Munich.
While publicly both club and player have stated a transfer for Lucas is not on the cards, Atletico are nevertheless wary of losing their star should an astronomic bid materialise.
One player who will be returning to bolster the capital side in defence is Spain international Jonny.
The 24-year-old, who can play on either side of the backline, is currently on loan at Wolves but will come back to Atletico at the end of the season, having made a positive impression so far in the Premier League.