The battle for a place in the top four and a spot in the next season&s European competition was starting to look like a race to the bottom as the teams chasing the prize stumbled.
It was there waiting for someone to grab the initiative and that's when the decisive move was made.
A spell of three defeats in a row, away to Crystal Palace, at home to Brighton and then at Saints, has left the club in fourth place with games in hand.
The old problems of a lack of battling quality, a tendency to choke when the pressure is on, were all brought up in the build-up to this game.
One win, however significant, does not answer all the questions, but this was a night when they showed they can do this again if they are to achieve their goals, and they must show they can do this again if they are to achieve their goals.
How they needed this and how happy they will be at the way in which they will cash in on the gifts given to them.
With a north London derby coming at the end of the year, it's no surprise that they're only behind Spurs on goal difference. What an event that will be.
Arteta's decision to let Pierre-Emerick Aubameyang go to Barcelona in January was coming under scrutiny with no replacement brought in.
He had to get something out of the bag and he decided to put the struggling Lacazette on the bench with Eddie Nketiah.
Arteta was rewarded for his predatory play as Nketiah scored twice, both times taking advantage of horrendous Chelsea defending, and received a standing ovation from the fans when he was replaced.
Can they gather the belief to make the top four?
That depends on which club comes up. It is likely that they will fall short if it is the team that has shrunk recently. They will have a chance if they play like they did here.
The foundation of a very good side here if they can recruit well in the summer is demonstrated by the fact that they got the right replacement for Aubameyang.
Ben White and Gabriel form a good central defensive partnership and they have two gems for the future, Emile Smith Rowe and Bukayo Saka.
The reasons why were on show here are the cornerstones that should be built on.
In the first half, he was outstanding, setting up Smith Rowe for the goal, creating further openings, and finally getting the goal he deserved to wrap the win up from the spot in the dying seconds.
This must be the result and performance that kicks off the season.
The defensive frailties that are creeping into their game is what made this another miserable night at the home of the Blues.
The FA Cup final and third place finish should be enough for the Blues, but they need to fix their defence.
They have conceded at least one goal in each of their last three games at the home of the Blues, with four of them coming against the league's bottom club.
Antonio Rudiger was out injured and Thiago Silva was on the bench for the first time.
In a terrible 45 minutes, the expected Barcelona signing sent a terrible backpass to Nketiah for the opening goal, before he was removed at half-time.
We come to Romelu Lukaku.
The signing of Romelu Lukaku from Inter Milan was supposed to be the final piece of the puzzle. He looks like a part of a puzzle.
He was laboured, lost too many physical and aerial challenges, and then he was also substituted. He has to produce soon but he doesn't have any signs.
This was the night when belief returned, the night when at least one of the top four teams woke up to the possibility that it is all still up for grabs.
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