... But it Could Take A While

EXCLUSIVE

Aaron Rodgers has a date with a 49ers jersey after losing a bet with the guys from Boyz II Men ... but the singers tell TMZ, they're willing to give him some time to prepare himself to pay up.

Singer Nathan Morris tells us, “Green Bay has another huge game [against the Bears] in 3 days so ... we understand where his head is, so I know he's focusing on that game.”

According to Nathan, Aaron promised to post the pic of himself wearing a 49ers jersey on his Twitter account if he lost against San Francisco ... which he did yesterday ... 30-22.

Nathan -- the biggest 49ers fan in the group -- is confident Aaron will hold up his end of the bargain, telling us, "Aaron might be one of the classiest guys we know.”