After a first half that felt frustratingly similar to the opening two games of the season, when the Baggies had plenty of the ball but lacked cutting edge, Diangana provided exactly the sort of spark that was missing.
His first goal evoked memories of Albion’s last loan star to score a screamer on league debut, and it was a 45-minute performance Harvey Barnes would have been proud of.
His second goal, an opportunistic diving header, showed a reassuringly predatory instinct that hadn’t been there before he was brought on.
All of a sudden, Albion had impetus, drive, pace, and power. Thank goodness Slaven Bilic called up former boss David Sullivan and pulled in a favour.
As Bilic said afterwards, it’s far too early to be getting carried away.
“We don’t expect him to score two goals every 45 minutes, but we can make that pressure be a positive one not a negative one,” said Bilic. “He won’t get carried away with this, it’s also part of our job to keep him humble as well.
“This season is a marathon, it’s not 45 minutes, his career is a marathon, not 45 minutes.”
But there’s no doubting Albion have got a talent on their hands.
It wasn’t just Diangana’s goals, which were two different types of finish, it was his ‘crazy’ take on the touchline and burst down the flank which ended with Charlie Austin hitting the post, it was his willingness to track back and make tackles on the edge of his box.
You don’t make 17 Premier League appearances for a team brimming with attacking talent like West Ham for no reason.
It wasn’t just Diangana either. Matheus Pereira was sparkling on the other wing and Austin added a threat up front.
Perhaps Luton had tired, and left more space because they were chasing the game, but the front three that finished the afternoon looked far more dangerous than the front three that started it.
That’s not to say Matt Phillips, Kyle Edwards and Kenneth Zohore won’t have a part to play this season.
The first two have already scored goals and Bilic was quick to make it clear that it could have been anybody who came off at half-time, not just Edwards.
They will all be needed, particularly in tight games like this when something is required off the bench. Phillips is still a valuable player at this level.
Bilic also complimented Zohore’s second-half performance, which was a slight improvement on his disappointing display against Millwall.
Zohore’s role will be to lay the ball off to the dangermen behind him like Diangana, Pereira, Edwards and Phillips.
But Austin looked more like scoring in his 10-minute cameo than Zohore did during the other 80 minutes, and the former Southampton man is surely closing in on a place in the starting XI, if not on Wednesday, then on Saturday.
That cutting edge up top was the missing ingredient to this team, which continues to control possession.
No team in the division has made more successful passes in the opposition’s half this season.
No player in the division has made more successful passes than Romaine Sawyers, Albion’s metronome at the base of midfield who is building a nice partnership with Jake Livermore.
Kyle Bartley is fourth on the list behind a couple of Marcelo Bielsa’s ballplayers. Albion are keeping the ball, and they are trying to dictate play.
They may not be the finished article, but at Kenilworth Road, there was further signs of development, and that’s all you can ask for at this stage of the season.
At first glance it looked like another poor header had been conceded, but on replay, Semi Ajayi slipped at the worst possible moment.
Bilic was more concerned Kieran Gibbs didn’t close down the cross quickly enough, something the left-back has been guilty of in the past, and probably stands in the way of a Premier League return.
The only criticism of the second half could be, in their eagerness to kill the game off with a third goal, the Baggies left themselves vulnerable on the counter.
But Bilic’s substitutes were positive, and they had the desired effect.
The three men he brought on all staked a claim for a starting spot once they are fit and ready.
There may have also been an extra special delight in beating Graeme Jones’s Luton side, if not for Bilic, but for those in blue and white ties in the directors’ box.
The board were unhappy he agreed to take the Luton job while still employed at The Hawthorns, but this was the Hatters’ first home defeat for nearly 18 months, a run which stretches back to their time in League Two.
That shows how difficult a place Kenilworth Road can be to visit, and not just because of traffic on the M1 and the limited parking spaces.
Nathan Ferguson was treated to a particularly hostile home crowd for having the temerity of being fouled three times by Callum McManaman, but the way the youngster dealt with it was mature beyond his years.
Seven points from a possible nine is a good start in anyone’s book, particularly because no team in the Championship is still boasting a perfect record.
But just like Diangana’s career, it’s dangerous to get too carried away. Albion won the opening two games of the season when they were relegated from the Premier League.
However, it is better to be up there at this stage than playing catch-up already.
And with more to come from some of Albion’s deadline day attackers – not to mention the return of Ahmed Hegazi and Gareth Barry – there is renewed faith for the season ahead.
It’s amazing what six minutes can do.