Derrick Henry is already one of the best running backs in the NFL, but Adam Sandler just gave him an incentive to go even harder on the gridiron ... 'cause the "Happy Gilmore" actor says if the Ravens star hits 2,000 rushing yards this season -- he'll give him a movie role!!

Henry was on the " Dan Patrick Show " on Wednesday ... and at one point in the convo, the two got to talking about Sandler, as the host was rocking a "Happy Gilmore 2" sweatshirt.

It turns out Henry is a big fan of the Sandman ... so Patrick laid out a challenge -- if he hits 2k yards this year, he'll have the acting great find a way to get him in his next movie.

Sandler got wind of the bet ... and not only did he accept those terms, he even threw in a dinner with King Henry, too!!

"I love ya, keep it

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