Diangana was sensational for the Baggies throughout the 2019/20 promotion-winning campaign.
But he has struggled for form ever since Albion agreed to sign the winger on a permanent basis from West Ham – in a deal potentially worth £18million.
Last Friday – just 24 hours before the Peterborough game – Ismael publicly told the 23-year-old his performances must improve.
And Diangana responded in style with the forward producing a impressive display from the substitutes bench that was capped with an excellent goal.
Now, though, Ismael says he has to maintain that standard – with Diangana potentially in line to start tomorrow's clash with Preston North End.
"It wasn't just the goal he scored, even before the goal his impact against Peterborough was brilliant," the boss said.
"He just got the reward with the goal, it wasn’t an easy one. It was a great feeling.
"We spoke last Friday and we said we know his quality, we know he can be a decisive player for us.
"Against Peterborough that was the case. But now he must keep going after this breakthrough and make sure that he can stay at that level."
Ismael was asked why he thinks Diangana played so well on Saturday.
“You need to ask him that question," the boss continued.
“Since I've been here we've worked with him very hard and we tried to give him all the support he needed and all the confidence he needed.
“We need all the players together and at a very high level of quality.
“Against Peterborough he took responsibility and we looked much better.
“If we are able to have 18 players every game at a high level we will look much better, we will win more games and we will be back in the race."
Meanwhile, new recruit Daryl Dike underwent a scan yesterday to discover the extent of the hamstring strain he picked up in the match against Peterborough.
The results of that scan are likely to be revealed today when Ismael holds a press conference ahead of the Preston game.