Mas was due to be on fourth official duties at Brammall Lane, but has is no longer available after citing an injury sustained at the Hawthorns.
Brighton defender Lewis Dunk scored a quick free-kick in the first half, which Mason disallowed before changing his mind and awarding the goal.
However, after VAR checked the goal, Mason had to blow his whistle again to confirm it had been ruled out, sparking angry reactions from the Brighton players.
Dunk described the decision to rule out the goal as "horrendous" and claimed the referee lost control of the game, which the hosts won 1-0.
"It's embarrassing, it's a horrendous decision," Dunk told Sky Sports. "I said to the ref 'can I take it?', he blew his whistle, and I took it.
"Just because there was so much pressure from the bench. Why doesn't he come and speak to the press like me? Never, they hide behind their bubble.
"I don't think he knew what he was doing. He gave the goal, why did he give it? I don't know why VAR was getting involved, he said 'goal'... you can look on the video if you want.
"Had he lost control of the game? Yeah, he did. Fact."
During the game, West Brom midfielder Conor Gallagher, on loan from Chelsea, collided with Mason, knocking the referee to the ground.
Additional reporting by Press Association.