NFL's Chad Henne -- Hey Jimmy Garoppolo ... Keep Your Head Up!

Backup QBs always stick together -- just ask Chad Henne who says Jimmy Garoppolo shouldn't be bitter now that he's lost the starting job ... saying, "It's the way the business goes."

Of course, Jimmy G was all set to cover for Tom Brady as the starting QB for the New England Patriots for the first 4 games of the season ... but that was BEFORE Tom's suspension was overturned.

But Henne -- who's the #2 QB in Jacksonville at the moment -- says Jimmy needs to stay positive ... and more importantly, stay ready.