Matt Damon broke his collarbone while mountain biking.
The 'Monuments Men' star snapped his clavicle four weeks ago after his bike got stuck on a bunch of rocks while he was riding down a hill and he tumbled over the handlebars.
He quipped: "Apparently I'm not a very good mountain biker.
"I just hit it [the pile of rocks] and literally the bike stopped, and I didn't. I just kept going."
The 43-year-old actor admits he may have been showing off in front of his friends while speeding down the steep hill, doing some tricks.
Speaking on 'The Tonight Show With Jay Leno', Matt - who was wearing a sling - said: "Yeah, like 'Watch this move and these turns.' And 'Watch me hit this rock'".
Despite injuring himself almost a month ago, the hunk didn't sport a sling during the Golden Globe awards on Sunday night (12.01.14) because it didn't match his tuxedo.
He explained: "I had that beautiful tuxedo on, and I didn't want to ruin it."
However, he was forced to put his stylish accessory back on at the after party because he was worried that his fellow stars may be a little heavy handed with his shoulder.
He said: "[I put it back on] so that no one came up and gave me the hail-fellow-well-met slap on the shoulder."