Napoli host the return-leg of their Europa League quarterfinal on Thursday needing to overturn a 2-0 deficit.
After victory over Chievo, Ancelotti said: “We were surprised by the Arsenal pressing game, as we couldn't play our way out from the back and kept trying to do it rather than attempt long balls. Their second half performance is more like the attitude they'll have here at the San Paolo, so going for the counter-attack.
“I chose Insigne and Mertens together because they are both quicker than Milik, so I wanted to use their pace. I think next Thursday the key will be to play a lot of vertical passes."