Big Time Rush's Logan Rushed to Hospital After Slicing Open Knee Onstage, See Pic

11:39 AM PT -- Logan Logan Henderson is now on the mend and he even returned to the stage to perform this past weekend.

Big Time Rush’s Logan Henderson was, uh ... rushed to the hospital in an ambulance after a big-time bloody injury he suffered onstage -- and it’s seriously graphic, so brace yourself before checking out the pic!

Sources with direct knowledge tell TMZ the band was in Krakow, Poland, on their sold-out European tour Thursday when things took a turn -- they slid down a set piece, Logan smacked his leg on something, and his knee just busted wide open. Big yikes.

This all went down toward the end of the show during one of their high-energy numbers, "Love Me Love Me" -- and the gash in his knee was so gnarly, the doctor had to close it with one giant suture to help it heal properly.

Logan’s bandmate Carlos hopped on the mic last night to let the crowd know Logan’s gonna be okay -- and that the show would keep rolling while he was getting treated.

This tour is one of Big Time Rush’s biggest productions yet -- they’re literally performing every song from every episode of their old Nickelodeon show -- so it’s no surprise Logan jumped on IG to reassure fans himself he’s doing A-OK.