It was noted on Wrestling Observer Radio that Riddle's official SmackDown debut could come as soon as this Friday as he performed on Monday and this week's SmackDown on FOX episode was taped then. It's possible that they filmed something that was taped for next week, but more than likely it was for this Friday's show. Next week's SmackDown was scheduled to be taped today.
Whether he debuts this week or not, multiple sources are reporting that the call-up is imminent. There have been plans to move Riddle to SmackDown since before WrestleMania 36, but those plans were put on hold coming out of the big event. The talk of finally bringing Riddle to the main roster started back up several weeks ago.
It's interesting that Riddle is not being called up to RAW to work under red brand Executive Director Paul Heyman as they have a history and are friends, and Heyman has been high on Riddle for a while. It was speculated on Wrestling Observer Radio that Riddle may have been kept off RAW due to a political situation with Brock Lesnar as Lesnar reportedly does not like Riddle. Furthermore, Bruce Prichard apparently wanted Riddle to come to SmackDown. Prichard reportedly wanted both Riddle and AJ Styles on the blue brand, and got them.
It was also indicated on Wrestling Observer Radio that WWE had to bring Riddle to the main roster because of his contract status as he would've been able to get signed anywhere he wanted, making more money than he has been making in NXT. The length of Riddle's original WWE contract was never confirmed, but it was reported in August 2018 that he signed a multi-year deal.
It's believed that this Wednesday's NXT episode will be the blow-off to Riddle's run on that brand, but that hasn't been confirmed. He is scheduled to face Timothy Thatcher in a "Cage Fight" with WWE Hall of Famer Kurt Angle as the special referee.