"It's another example that football changes quite quickly," Potter said of the 21-year-old's crucial contribution.

"It can feel that you are not in the team or in the group. But you are always in the group, you're always available, you're always working hard to try to wait for your opportunity so when that comes you are ready.

"Alexis has been a top professional and he's worked hard, he's got on with his job, helping the team, encouraging the team, and it's nice.

"That's the role of the subs sometimes. Everybody wants to start, but sometimes it's more important that the guys that finish the game can influence it and get the result we need."

Albion dominated proceedings in south London but looked set to return to the south coast empty handed until Mac Allister's late intervention.

Rivals Palace had led from the 19th minute thanks to Wilfried Zaha's penalty – their only attempt at goal all afternoon – awarded for Tariq Lamptey's adjudged foul on Michy Batshuayi.

The contest ended on a sour note for the visitors as captain Lewis Dunk was dismissed in added time for a reckless lunge at Gary Cahill.

Former England defender Cahill was left in clear discomfort following the two-footed challenge on his first appearance of the campaign following injury.

Eagles boss Roy Hodgson was delighted to welcome back Cahill and praised his leadership qualities.

"First of all, he's a very, very good defender. His positional play is very good, he's exceptionally good in the air, he's a strong challenger for the ball and a very good blocker of the ball," said Hodgson.

"These are the defensive qualities that we don't talk about every day because they aren't the so-called sexy qualities. They are the qualities that mean you can keep clean sheets.

"He's also brought us leadership qualities, he's been very good leadership-wise.

"He's the best type of leader; he's the quiet leader that people don't see from Row Z in the stand. He's the leader that people see on the training field week to week."