Six years after their debut Europa League final appearance, Chelsea are back for a second taste of UEL glory. The Blues clinched their lone championship in the competition through a 2-1 win over Benfica in 2013. Now they have a chance at winning their second Europa League title, this time with an opportunity to best their London-based rivals Arsenal in the process.

To get to this stage, the Blues had to defeat Eintracht Frankfurt in the semi-finals. Chelsea tied with the German club 2-2 on aggregate, with both sides splitting 1-1 draws across the two legs. Following a scoreless extra time, Chelsea emerged victorious through penalties by a score of 4-3. Kepa Arrizabalaga made two big stops on Martin Hinteregger and Gonçalo Paciência, and Eden Hazard scored the winning penalty.

There was a moment in extra time when Chelsea had seemingly scored the winning goal, though. In the 115th minute, Eden Hazard sent a high ball into Eintracht Frankfurt’s crowded box. The cross went straight to goalkeeper Kevin Trapp, who looked to have it under control. But the ball slipped from his grip and ricocheted off of Chelsea’s César Azpilicueta—who had collided with Trapp—and into the back of Eintracht Frankfurt’s goal. Advantage Chelsea, supposedly.

Azpilicueta lucky to even be on the pitch, maybe a bit unlucky on this one? Letter of the law, think ref got this right but we’ve seen this goal given. #CHEFRA pic.twitter.com/9YwVlDHYKp — Anthony DiCicco (@DiCiccoMethod) May 9, 2019

Referee Ovidiu Hațegan, who initially looked to have signaled a goal, called it back for an Eintracht Frankfurt free kick. He determined that Azpilicueta had interfered with Trapp’s ability to catch the ball properly, and as a result disallowed the goal. With no VAR in use, he didn’t get a second look at the incident.

What if he did, though? Would he have still called it back? To understand that, we must first understand what the official football rulebook says about incidents like this.

On page 103 of the Fouls and Misconducts section in their 2018-2019 “Laws of the Game” book, the International Football Association Board explicitly states that a “goalkeeper cannot be challenged by an opponent when in control of the ball with the hands.” In other words, if a goalkeeper has complete control of the ball, an opposing footballer cannot attempt to take the ball away from them. If they do, they’re adjudged to have fouled the goalkeeper and a direct free kick is awarded to the ‘keepers team.

But how does the IFAB define “control of the ball”? On the same page, the IFAB considers a goalkeeper to be in control of the ball if they meet one of the three criteria; if they’re “bouncing it on the ground or throwing it in the air”, if they’re “holding the ball in the outstretched open hand”, or if “the ball is between the hands or between the hand and any surface (e.g. ground, own body) or by touching it with any part of the hands or arms”. The only exception to the third criteria is a loose ball off of a rebound.

Now that we have the rules down, let’s look at the actual incident. At his highest point, it looks as though Kevin Trapp has complete control of the ball (see first image below). Both of his hands are on the ball, and he seemingly has the situation under control. As he descends though, Trapp loses control (see second image below). He allows the ball to drop freely while keeping his hands close by. This was likely done to improve his grip; he didn’t look to have a firm catch on the ball initially. In trying to readjust though, he loses control of the ball, allowing it to rebound off of Azpilicueta and in.

With this in mind, along with the IFAB’s rules, surely this means that the goal should’ve stood, correct? After all, it’s a ball that could be defined as not being controlled by the goalkeeper, therefore allowing Azpilicueta to challenge for it, right?

Well, not exactly.

Upon closer inspection, while Trapp does let go of the ball for a moment, Azpilicueta is the reason why Trapp couldn’t reset his grip on the ball properly.

Pay attention to Azpilicueta’s right arm below.

Notice how Azpilicueta’s right arm interferes with Trapp’s left arm as he challenges the goalkeeper. Azpilicueta’s right arm prevents Trapp’s left arm from descending with his opposite arm, resulting in an awkward catch where the left hand is on top of the ball and the right hand is on the bottom. Had Azpilicueta not interfered, it’s likely Trapp would’ve readjusted properly and caught the ball. But because of Azpilicueta’s right arm interfering with his left arm’s movement, Trapp could not properly catch the ball. Considering this, the referee was correct in disallowing the goal.

If that wasn’t enough, Azpilicueta’s momentum carries him into Trapp’s right arm, pushing it away from the ball. This also plays a major factor in why the ball escapes Trapp’s grasp. With his right hand being forcefully pushed away from the ball, Trapp can’t fully control it and it drops into play. Considering the fact that Trapp’s ability to catch the ball is being hindered by Azpilicueta’s challenge, the goal must be called back.

This incident didn’t matter in the grand scheme of the game. Chelsea would win in a penalty shootout thanks to heroics from their own goalkeeper, Kepa Arrizabalaga. But suppose the referee had an opportunity to take a second look at this called-back goal. Considering the fact that Azpilicueta interfered with Trapp’s ability to properly catch the ball, I believe the ref would’ve stuck with his initial decision and given a direct free kick to Eintracht Frankfurt.