The France international scored for the first time in 45 club matches in a run stretching back to April 2019 with his finish from a tight angle in Saturday's LaLiga clash.
Lemar's rare strike gave Atleti a platform to build on as they wrapped up a seventh straight league win through Marcos Llorente's well-taken goal.
Simeone has regularly defended the form of Lemar in recent weeks and hopes the 25-year-old, a €60million signing from Monaco in 2018, can now rediscover his confidence.
"The goal makes me very happy," he told Movistar. "He has been working for a long time to show what a footballer he can be and what he possesses inside.
"Hopefully this goal will help generate some confidence so he can keep his place in the side and reach his goals for the club.
"There are many games to play so the better the group is in general the better we can compete."
As well as scoring Atleti's important breakthrough goal 11 minutes into the second half, Lemar also created more chances (three) than any home player.
Lemar insisted he has never questioned his own ability and is grateful to have received Simeone's backing.
"I am very happy to have scored. It was important for me because it's been a while since my last one," he said. "Most of all I'm happy for the team because we played well.
"I have no doubts about myself – I know my quality. Criticism is a part of football so you have to have a strong head.
"Simeone knows my quality and he always wants to bring out the best in me. Today was good for me and for the team, but now we have to keep on working hard."
Atletico managed just one attempt on target in a first half that Valladolid, previously unbeaten in three games, edged at the Wanda Metropolitano.
Llorente's introduction at half-time, plus the arrivals of Koke and Joao Felix before the hour, made a huge difference as Atletico moved top of LaLiga with a sixth home league win in a row in this fixture.
Luis Suarez returned to Atletico's starting line-up after missing their last four matches with coronavirus but struggled to make much of an impact in his 63 minutes on the pitch.
The 33-year-old wasted a big chance early on when failing to round Jordi Masip and found the target from just one of his five shots, but Simeone expects the striker to soon regain his rhythm.
"Suarez has been out for two long weeks, since playing against Ecuador, so it has been a while since he was last on the field," Simeone said.
"He played longer than I expected. I'm sure those minutes will help him for what is to come because we need a lot from Luis."
Atletico, still yet to trail in a single LaLiga game this season, head to Real Madrid next weekend following Wednesday's trip to Salzburg in the Champions League.