The French winger scored a hat-trick to guide Atletico to a 4-1 triumph at the San Mames stadium on Sunday.
Atletico, without team’s top scorer Mario Mandzukic through suspension, trailed 1-0 at half-time before Griezmann stole the show after the restart.
“In the first half he (Griezmann) could not find a way to help out the team, but the second half was just the opposite. He was fabulous,” Simeone told the club’s official website.
“He is a player we need, that we went looking for. We are very happy for him, as we trust a lot in his characteristics and we need him.
“This performance will be good for his confidence. He has always delivered when given a chance.
“It shows his work is paying off. He gives us something different.”
Griezmann joined Atletico in a 30million euro deal from Real Sociedad this summer and has scored six goals in 16 league appearances so far.
The victory, Atletico’s 11th in La Liga, allowed Simeone’s side to end a fantastic 2014 in the best possible way.
Atletico, who won La Liga last season for the first time in 18 years, are third in the standings on 35 points heading into the Christmas break.
They trail four points behind league leaders Real Madrid, who have a game in hand, and are three adrift of second-placed Barcelona.
Atletico are also alive in the Copa del Rey and the Champions League, where they will face Bayer Leverkusen in the last 16 of the competition.
“It really wasn’t easy to come to San Mames and win,” Simeone said. “But my players reacted and allowed us to end the year on a positive note.
“It has been a fantastic year and I have to say thank you to my players for what they have done.
“It has been extraordinary in terms of numbers and we can still improve next year.”