Jake Paul just gave Deji -- the guy he's fighting in Manchester -- a Rick James welcome to his fan meet and greet ... introducing his 5 fingers to Deji's face, SLAPPIN' THE HELL OUT OF HIM!!
The slap to Deji's face all went down when KSI's little bro crashed Jake's gym ... where he was meeting up with fans for autographs and pics.
The YouTube star walked in -- talked trash -- and then had his face smacked to the moon by Logan Paul's baby bro.
"Deji showed up at my gym unannounced to try & talk shit," Jake said of the altercation.
"As soon as he walked in I slapped him in the face like the Bitch he is & then he left like a bitch..."
Fight night is just three days away. Thoughts and prayers go out to Deji in the meantime.