Simeone named Joao Felix, Koke and Marcos Llorente among the substitutes with key games against Salzburg and Real Madrid to come over the next week, and Atleti struggled to get going as a result.
Llorente was introduced at half-time and Joao Felix and Koke followed before the hour, though Lemar had made the all-important breakthrough a few moments earlier with his first club goal since April 2019.
Atleti were in control from that point and added a second through substitute Llorente's route-one strike as the leaders made it seven league wins in a row to move above Real Sociedad.
Jordi Masip spilled Angel Correa's long-range shot but Luis Suarez could not take advantage four minutes into his return, miscontrolling the ball as he attempted to round the keeper.
The visitors entered the game unbeaten in three and looked the more threatening in the first half, with Jan Oblak keeping out efforts from Oscar Plano and Marcos de Sousa.
Simeone's men were given another scare four minutes into the second period when De Sousa's volleyed shot at the back post was blocked by Kieran Trippier in front of goal.
But Atletico grew into the game and, a few moments after Correa fired wide on the turn, Lemar converted Trippier's cross from a tight angle after Suarez failed to reach it at full stretch.
That ended a run of 45 games in all competitions without a goal for the France international and Atleti had a second 16 minutes later - Trippier flicking on Oblak's pass for Llorente to race through and tuck past Masip.
What does it mean? Positive start to big week for Atletico
Atletico need to avoid defeat away to Salzburg on Wednesday to advance to the last 16 of the Champions League, before then travelling to bitter rivals Real Madrid in the league three days later.
Simeone was therefore keen to give a few of his key men a breather today but that did not go exactly to plan, as Joao Felix, Koke and Llorente were all brought on to help break down a resolute Valladolid side.
Atleti had only one shot on target in a below-par first half compared to three in the second, two of those beating Masip, as they took control of the game and made it six home league wins in a row in this fixture.
Kieran the key man
Trippier blocked De Sousa's goal-bound volley with the game delicately poised at 0-0 and then set up both of his side's goals, with no player on the pitch touching the ball more times than the right-back (93).
On what was his 48th appearance for Atletico in all competitions, he became the first English player to set up two or more goals in a LaLiga game since David Beckham for Real Madrid against Villarreal in May 2006.
2 - Kieran Trippier has provided two asissts in a single game for the first time for Atlético de Madrid in all competitions (48 apps), becoming the first English player to do so in a LaLiga game since David Beckham against Villarreal in May 2006. Stellar. pic.twitter.com/FkWpX8jOuj— OptaJose (@OptaJose) December 5, 2020
Suarez's minimal impact
The Uruguay international had missed Atletico's previous four matches after testing positive for coronavirus while away with his country last month and was understandably rusty on his return to action.
He squandered a good chance early on and found the target from just one of his five shots before being replaced a little over an hour into the match.
Key Opta facts
- Atletico Madrid have only conceded two goals after their first 10 LaLiga games this season, the joint-lowest figure at this stage of a single campaign in the competition's history (alongside Deportivo La Coruna in 1993-94).
- Atletico have kept a clean sheet in eight of their last nine LaLiga games, as many as in their previous 22 matches in the competition.
- Atletico are the only LaLiga side that have not trailed for a single minute this season.
- In the last two LaLiga seasons, only Sevilla's Jesus Navas (10) has provided more assists in the competition without scoring a single goal than Atletico's Kieran Trippier (seven).
- Atletico's Marcos Llorente has been involved in four goals in his last four LaLiga games at the Wanda Metropolitano (three goals and one assist).
Atletico have those big games with Salzburg and Madrid over the next seven days, while Valladolid host fellow strugglers Osasuna on their return to LaLiga action next weekend.