After receiving a first-half booking, Rice was shown a second yellow card shortly after the break despite appearing to be kicked by Joel Veltman when the Brighton defender went to take a free-kick.
Replays showed Rice toed the ball away from Rice just before he tried to take the set-piece, but it still looked a very dubious call, and Morgan was having none of it.
Taking to X, the Arsenal super-fan fumed: 'What the f*** was that? Declan Rice sent off for being kicked to pieces? Worst red card decision I’ve ever seen.'
Morgan's mood was not helped by Joao Pedro cancelling out Kai Havertz's first-half goal just nine minutes after Rice's red card.
Declan Rice was shown a bizarre red card early in the second half against Brighton
He followed up with a further X post that read: 'And now, of course. Brighton score.. what a pathetic joke.'
It was the first red card of Rice's career, and completely changed the momentum of the game after Arsenal had taken control in the first half.
Bukayo Saka outmuscled Lewis Dunk before playing a through ball for Havertz who coolly lobbed the ball over Jason Steele in the 38th minute.
Arsenal captain Martin Odegaard had earlier missed a good chance as well, as the Gunners threatened to build a dominant lead.
But Brighton stayed in the game and then had the debatable Rice decision go in their favour to help their cause.
The Seagulls pushed for a winner against their 10-man opponents, but Arsenal held on for a draw.
However, it was still their first dropped points of the season, and the crowd and Arsenal's players were left furious at the final whistle.