Napoli lost 3-1 at Barcelona to tumble out of the Champions League round of 16 4-2 on aggregate.
Capello told Sky Italia: "Napoli's defence was too submissive, not very aggressive. With the quality of Barcelona, if you're not careful, if you're not on the man, they go to goal.
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"Napoli's defence was lacking. Osimhen? In the first half he went six times offside, in the second half we saw a little more but in general we saw little. Cubarsi suffered, he was never in control of his body. Barcelona's defence is the weak point and what they don't do is highlighted.
"I am not for penalties, I'm not for penalties, but the referee wasn't 50/50 and the foul on Osimhen should have been sanctioned with a penalty kick."