Shelvey tried to act as peacemaker when punches were thrown, but inadvertently got caught in the crossfire.
The incident has left the midfielder wearing a cast, but that will not keep him out of next weekend's trip to Southampton.
Lascelles reportedly accused Diame of being 'lazy', starting the confrontation on Thursday.
The news comes just 24 hours after the newcastle captain signed a new six-year deal at the club.