Chelsea's defence is their main problem right now and if Jose Mourinho and his men want to get out of this lull they need to sort it out and fast.
Chelsea players went into the game with Porto well prepared for the physicality of their opponents and their aerial threats, as expressed by Diego Costa after the match.
"We had the game under control. We knew it was going to be a physical game. We began well. Things changed when they scored, but we quickly equalised. In the second half the same thing happened - we were lacking concentration at key moments of the game."
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This lack of concentration produced the first goal as Yacine Brahimi out-foxed Branislav Ivanovic to create the goal.
Speaking about the second goal, Mourinho said; "For us it's a ridiculous mistake. We watch dozens and dozens of repetitions of Porto taking corners.
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"I thought we were completely ready for that. And in a moment when the game was under control and we were preparing a change, to concede that goal was like ice in the players.
"The basic thing in football to work at is a defensive set-play. The ball is stopped, you defend the spaces or the man or both. We had another similar mistake (from a corner) when they hit the post."
Trouble at the Bridge
Chelsea has now suffered five defeats in 11 games in all competitions this season. They are struggling to defend their Premier League title and Mourinho's troubles appeared to have spilled over into Europe.
But the Portuguese manager and his men must put a stop to this. It's a good thing they know the problem. The defence is just making too many mistakes and this is so uncharacteristic of Chelsea.
When the defence is porous, the team suffers. The attacking players begin to play with fear so as not to lose the ball and expose the defence and in so doing fail to play with the initiative.
The defensive players suffer a crisis in confidence and become jittery when the opposing attacker storms forward, while the opposing player increases in confidence and seeks every opportunity to run at his man.
If Chelsea are to turn their season around, then they must first create an impenetrable back line even if it means sacrificing some attacking play and Mourinho being the best in defensive displays will definitely get it right.
Chelsea fans: Is Mourinho the man to turn it around, or should another manager be considered? Give YOUR opinion in the comment box below!