As noted, WWE Universal Champion Brock Lesnar was scheduled to face off with Roman Reigns on Monday's RAW from Anaheim but that segment never happened. Lesnar, who was in Las Vegas on Sunday to meet with UFC President Dana White but not to attend the WWE Elimination Chamber pay-per-view, was listed in multiple RAW scripts on Monday and was still being advertised by WWE after RAW hit the air.
It's still not completely clear why Lesnar and Heyman didn't appear but F4Wonline.com reports that their absence is believed to be due to an 11th hour creative change. The goal of the build to Reigns vs. Lesnar at WrestleMania 34 is to get Reigns cheered by fans, leading to a positive reaction when he wins the title in New Orleans. WWE officials also want the build to Reigns vs. Lesnar to come across like a shoot as much as possible, and the RAW segment helped with that goal.
We also noted before that the RAW segment with Reigns, Lesnar and Paul Heyman was scheduled to be lengthy and would air in the second hour. PWInsider adds that the segment was scripted to have Lesnar call Reigns a "b---h" at one point. Regarding the promo that Reigns did by himself, word is that the promo was not in the script but most of the verbiage was approved, likely by Vince McMahon.
Lesnar is scheduled to defend his title against Kane at Saturday's WWE live event in Chicago. He's scheduled to return to TV at the March 12th RAW in Detroit.