2:15 PM PT -- Cops determined Millsap was NOT at fault ... according to a police report obtained by Chris Vivlamore of AJC.com.

Vivlamore says cops cited the biker for failure to maintain his lane.

12:20 PM PT -- Good news ... the Hawks tell TMZ Sports that Millsap was not injured and he even worked out with the team Tuesday.

Atlanta Hawks superstar Paul Millsap was involved in a serious accident involving a motorcycle -- which left his Rolls-Royce smashed up and the bike wedged into the front end ... TMZ Sports has learned.

Here's what we know ... it all went down around 6:30 PM on Monday in Atlanta. We've spoken with several people connected to the incident but don't know exactly who crashed into whom.

We're told the biker was injured in the wreck and was transported to a local hospital in serious condition.

Officials at the scene say the motorcycle was on fire and the ATL Fire Dept. responded to put out the flames.

As for Paul, all we know is he was in the Rolls-Royce at the time of the crash -- unclear if he was the driver or a passenger.