First off, Cech threw the ball towards Eto’o to start off a counter-attack. However it was quite a poor throw, and Everton were able to capitalize and recover despite Ramires trying to get the ball back. The three players encircled had a direct effect on the goal Chelsea conceded right before half time.

With 3 players indulged in recovering the ball from the flanks, there were two Everton players who were quite free. Eto’o at this point was nowhere near them, and the responsibility of pressurizing them fell on Schurrle’s shoulder. Everton were clever while going forward by letting Chelsea’s high pressing fall on the wrong players and 2-3 midfielders were smart enough to take advantage of it.

Everton cut the ball back, and here is when the disaster becomes worse. Mikel is forced into pressurizing the ball because if he doesn’t, the ball is played out somewhere else with more attacking intent. Luiz, however, breaks  away and tries to press at the ball as well. This was the main reason behind Everton’s goal. Luiz is ball watching, something you don’t expect from world-class defenders. What Luiz should have done is held his ground and let Schurrle who is coming in as well, to press when the ball is passed into the middle and towards Barkley.
With Luiz’s movement, the defensive line was messed up. 2 defenders were now marking 2 instead of 3 marking 2. The numerical advantage was reduced. If we were to have 3 throughout the period, the ball could have been cleared off well right when Jelavic headed the ball back into the middle and at Naismith. Also note how JT is far off from Naismith, as even he couldn’t have predicted Luiz to have just walked away from his responsibilities to press at the ball needlessly.

Another thing to note is how Barkley had brilliant vision to play it towards the flanks where Osman is making a run, despite being pressurized by almost three to four players. Jelavic has his hand up as Terry closes in on Coleman who’s made the run inside, and Ivanovic marks Naismith.
This is where the effect of Luiz’s ball watching can be clearly seen. Everton take good advantage of Chelsea’s high pressing thanks to Coleman making the decisive run which turned the numerical advantage to their favor . From 3 on 2, to 2 on 3 in a matter of seconds for Chelsea’s defensive line.

Cole did a pretty poor job of blocking off Osman. Even though Ramires was a lot more closer to the Everton midfielder, it was never really his job to do it. Luiz has realised his mistake and is slowly making his way back towards the defensive line that was once well structured, but the damage has already been done. Ivanovic has to rush back in time to stop Jelavic from getting in contact with the ball.

Jelavic pulls it back as Luiz continues retreating. Terry is forced to mark two players, but realizes that the main danger is Naismith. He is forced to press on him, but can’t make it in time.
Technically speaking, even if Terry was to clear the ball away, Coleman could have put the ball in the net. Not only were Everton able to create a direct scoring chance, they also had a plan B.

This gives a much clearer view on the problem. The fact is that Luiz should never have moved, cause Schurrle was always going to be the one pressing on Barkley. Luiz could have stayed and maintained the structure, instead his adrenaline forced him into pressing and Chelsea paid the price.
Apart from the goal, we can have a look at Mikel, who put in a MOTM performance. He looks like one of the only few players who will fit into Mourinho’s system with ease. The defenders have to press with a definite structure, the full-backs have to learn when to initiate counter-attacks and when not to, the midfielders including one player from the pivot needs to understand that they have to track back and press efficiently while conserving enough energy to attack as well and the striker needs to be able to drop deep and link up well. The defensive midfielder however has to press, break up plays, defend area rather than man mark (zonal marking) and also help in maintaining possession.
Mikel seems to easily fit in while others have to train to fit into their roles, and this is why he ended up as one of our best players on the pitch. The problem is that he has to deliver consistent performances, and only time will tell if he can. Ramires and Mikel had good communication between them, and that was well evident.
Looking at few passing graphs –

66/71 passes completed, He had a pass completion rate of 92.95% . The failed ones where mostly long balls. He basically was the person in the middle who players went to when they wanted to be relieved under pressure. He is the kind of player that will thrive when facing a side put up by Martinez.
To summarize, there isn’t much we have to be worried about. We created chances, were able to link up well but only need to work on a few mistakes and pray that luck is on our side. The Luiz conundrum, however, is something that can’t be resolved easily.