A star of the team that won promotion in the 2019/20 campaign, Diangana has found it hard to reproduce that sparking form ever since joining the Baggies on a permanent deal last summer.
Injuries together with a dip in confidence have ensured the 23-year-old hasn't impacted the Premier League in the way he would have hoped this season.
But with the Baggies relegation to the Championship having been confirmed, Allardyce gave the winger his first start since January 2 in the narrow loss to Liverpool on Sunday.
And the boss believes it's important for Diangana to get minutes now in order to ensure he gets back to his best for the start of next season.
"Grady has been fit for quite a while but we've been playing the same team on a regular basis," Allardyce said.
"We’ve been very good with injuries, so we’ve not had to change the side.
"But I felt it was the right time. We’re not going to be affected by our position in the league, so we can let him get out there and see what he can do.
"Against Liverpool he contributed well and it was very pleasing to see that.
"Obviously we’d have liked to have seen more of his ability in the final third, but that will come when we’re playing a lesser team than Liverpool, because against them you have to do the defensive side right.
"All the team, the players, did the defending side brilliantly. When they could, they went off on the attack and tried to create as many chances as they could. I was pleased for him."
Allardyce's future at Albion remains very much up in the air with the boss having a break clause in his contract that has now become active following the club's relegation back into the Championship.
The 66-year-old has made it clear he will confirm whether or not he will be in charge next season before the game against Leeds on Sunday.
But the passion he showed on the touchline against Liverpool, together with his inclusion of Diangana, has led to speculation amongst supporters he will stay.
Allardyce, though, says people shouldn't read too much into things – with the boss insisting still hasn't made up his mind on whether to carry on or call it a day.
"I get wound up with the emotion of every single game no matter where it is and no matter what it means – whether it keeps us up or whether we’ve been relegated," he said.
"I expect the players to perform to their best and give their all so I need to perform to my best."