WWE confirmed that Rey Mysterio vs. Roman Reigns will happen inside the titular Hell In A Cell structure on the latest episode of SmackDown.
Rey fired out the challenge to Reigns, then the pair brawled. It did look like WWE were heading for some sort of Roman vs. Rey bout on the pay-per-view due to recent events, but the fact they're placing both inside the cage is a bit of a surprise.
HIAC 2021 happens next week (20 June).
The company didn't leave themselves a lot of time to work with here - WWE are clearly relying on Mysterio's underdog status and Roman's arrogance to sell the showdown. It's all a bit last-minute though. There's no getting away from that.
In fairness, Reigns had nothing else going on. The only other scenario open to WWE was putting him inside the Cell with his cousin Jimmy Uso; it looks like SmackDown's writers want to let that clash simmer for another month or so instead.
Roman vs. Jimmy may end up happening at Money In The Bank on 18 July, or even at SummerSlam come 21 August.
For now, it's Rey vs. Reigns at HIAC.