Atletico looked to be in danger of dropping their first points of the season on Sunday, having beaten Getafe in their season opener, but substitute Vitolo finished coolly 19 minutes from time to seal the victory.
Alvaro Morata was one of few players to look even remotely threatening in the first half, with proceedings not offering much excitement until the second half, which saw a flurry of action early on.
Both sides struck the frame of the goal and Joao Felix wasted a glorious opportunity, but the €120million signing made amends by setting Vitolo up for the winner.
1 - João Félix (19 years and 289 days) is the youngest player to provide an assist in LaLiga this season. Premiere. pic.twitter.com/3KZr69iALW— OptaJose (@OptaJose) August 25, 2019
Atletico shaded a generally drab first period, with Morata the only player to go close.
He first shot wide from Koke's low cross in the third minute and half an hour later latched on to Kieran Trippier's lofted ball only to be thwarted by the approaching Juan Soriano as he attempted to prod an effort over the goalkeeper.
The second half began in much livelier fashion – Joao Felix spurning a glorious chance in the 50th minute when needlessly attempting a scissor-kick from 12 yards with the goal gaping.
Leganes went even closer soon after when Jonathan Silva's free-kick struck the crossbar, before Koke nudged a Morata cross onto the post.
What does it mean? Simeone's tactical alteration does not backfire
Simeone opted to deploy a back three, a change from the usual four-man defence. While it might have seemed a strange decision given they were expected to win anyway, it certainly did not hinder Atletico.
After all, the free-kick aside, Leganes rarely threatened. Given Atletico's numerous centre-back options this season, we may well see more of that back three.
Vitolo a driving force
Introduced for Mario Hermoso after an hour, Vitolo had a fine impact off the bench. The winger has had a difficult time since joining from Sevilla, but he looked more like his old self on Sunday, getting into scoring positions and worrying the defence with some driving runs.
Omeruo invites pressure
While Dimitrios Siovas mostly looked solid at the back for Leganes, Kenneth Omeruo was anything but. The Nigerian centre-back made several mistakes that put his team on the back foot and he often appeared shaky.
Atletico return to the Wanda Metropolitano next Sunday to host Eibar. Leganes go to Real Betis a day prior.