The France international arrived in a €30 million deal from Real Sociedad at the start of the season and took time to settle, attracting criticism from the Spanish press as a result.
However, the 23-year-old has since become an integral part of the Atletico attack alongside Mario Mandzukic, scoring eight goals in his last six La Liga outings.
Griezmann fired Atletico into a seventh-minute lead against Eibar at Ipurua on Saturday, before setting up Mandzukic – who went on to score twice – for their second in a 3-1 win.
Simeone told reporters afterwards: "It would be unfair to assess him [Griezmann] for all the effort he has made until now. We've talked and worked hard with him.
"What he is giving us today is what we need for the players of Atletico Madrid. It is difficult to play here only with talent.
"You need talent, but more effort and that's how we can compete against the best. We tried to go in search of it [extra work ethic] - he has it and is a decisive player for us."
Atletico claimed victory in testing circumstances, with their arrival in Eibar coming amidst a torrential downpour, leaving the pitch saturated.
But Simeone showed the presence of mind to pinpoint a particular area of the surface that appeared to be less affected, and he subsequently deployed Griezmann there with devastating results.
"We played a very good match right from the start," he added. "We sought to attack with Griezmann down the good side of the pitch and avoid the area where the ball was not moving well, as his speed when one-on-one can hurt.
"They [Eibar] could have got into the match, but a good stop by [Miguel Angel] Moya avoided that. I think they were equal to us in the first half, but I think we controlled it very well in the second half and we got a very important victory.
"It was a difficult pitch and was not easy to play. We had very clear mind of what we wanted to do and it went well."
