This season has started badly for McLaren and after four races Button and Perez have scored just 13 and 10 points respectively, putting them both more than 60 points behind leader Sebastian Vettel.
The team does not fully understand their new car, which has had major changes to last season's machine in a bid to give greater development potential, and so far they have been playing damage limitation.
But the experienced team has proven time and again that they are kings of the comeback and they will bring a major new upgrade to Barcelona in an effort to steer themselves back on course.
If that works, things could start to get more serious – which is why Perez's performance in Bahrain was so crucial.
Button leads Perez in Bahrain (AFP)
When you arrive in a top team having made your mark in less competitive machinery, as Perez did with Sauber, life is not easy. You can't expect to be as fast as your more established team-mate straight away, yet F1 is such a fickle and competitive environment that you need to show you can step up a level quickly before question marks begin to be raised.
Perez had been going about things the right way, settling in, keeping out of trouble and learning how to work within the team.
In Australia, his first race weekend for the team, a gamble on slick qualifying tyres went wrong and saw him start down in 15th place - but he showed good race pace and finished just two seconds shy of Button.
In Malaysia, he was almost a second slower than Button in their first genuine qualifying 'competition' but that only cost him two grid spots. He had technical issue on car in the race but still scored points.
In China, after crashing in the pit lane in practice he composed himself and halved the deficit to Button in qualifying to 0.5s. He was fast again in the race but was out-raced by Button, finishing half a minute down.
His first three races were nothing spectacular, but then there was nothing too wrong either. But still his approach, confidence and attitude were called into question, like an underperforming striker in a football team struggling for form.
Team boss Martin Whitmarsh said Perez lacked aggression, saying: "You have to use elbows and you have to be robust without being dirty."
It was the best thing Whitmarsh could have said.
In Bahrain, we saw a different Perez. Elbows out, hustling, attacking and racing like a man whose future depended on it. At times he didn't quite follow the "without being dirty" part, but as Whitmarsh said afterwards, that can be worked on.
He stood his ground where he had previously given way in hard fights with Romain Grosjean and Fernando Alonso and although his serious wheel-to-wheel combat with Button stepped beyond boundaries of the racing code, what aspiring future star hasn't done that in the past?
Yes, it was with his team-mate, but if the team allows their drivers to race, as McLaren did, then why is that any different to battling with any other driver?
The drivers at McLaren's launch (Reuters)
What matters is that this change in approach helped Perez beat Button, and by some margin. He finished four places ahead, 11 seconds up the road, having started two spots behind.
Perez had better pace than his team-mate because he was able to manage the overheating of the rear tyres better – the key to a strong performance on the new Pirellis.
And that could be a worry for Button.
Tyre management is usually one of Button's trump cards and this year's McLaren has been designed to capitalise on his soft-touch driving style. But Perez proved with the Sauber last year that his equally smooth driving style means he's also adept at conserving tyres. So the McLaren suits him too.
Perez has shown a fast-growing maturity and before Bahrain he spent significant time with his engineers to pre-plan their approach to the race weekend. He has worked to improve qualifying consistency and quickly got an understanding of how to work the new Pirelli tyres.
Sure, he overstepped the mark in the race, but back in the midfield and upper midfield, where he spent his first two seasons in F1, that kind of wheel-to-wheel aggression happens more often, particularly in the argy-bargy bustle of the start - yet goes relatively unnoticed.
After the battle, wise old Button said: "That's the sort of thing that happens in karting, but most people grow out of it..."
It's a fair point. Perez is 10 years Button's junior, with just 41 starts compared to 232. And it did show the Mexican's inexperience - but it also showed the reason why McLaren signed him.
"You don't want to extinguish the spark," said Whitmarsh following a head-to-head with the two drivers after the race. "We can focus on some of the things that weren't perfect today...but on balance it was the right thing in the long-term for both drivers to know they are racing each other and be prepared to."
So that's that then. It has been declared an open fight at McLaren and although it's all amigos together now away from the heat of battle, Perez is not about to sit back down.
He has a multi-year contract, but there will be break clauses if he doesn't meet expectations. He knows he has just one year to shine.
"He needed this race," said Whitmarsh. "People were starting to get on his case, expecting to see a bit more of a spark. That is what got him past Alonso and Webber and others. He has demonstrated the passion and spark people expect to see from him."
Yes, he needs to find that balance between over aggression and a fair fight, but Bahrain showed he is not a pushover – and it also showed he has the pace and tyre control to get the best out of the McLaren.
Button's reaction was fair, but you can't help but think it came partly because he knows that if and when he finally gets a car to race at the front, he's not going to have it all his own way.