The Pole should have put Napoli ahead minutes later when compatriot Piotr Zielinski’s cross found him unmarked in the penalty area but he headed wide.
Arsenal also lost Aaron Ramsey to an apparent hamstring problem but, just as it seemed things were going against them, they went ahead as Lacazette curled his free kick around a poorly organised wall from 25 metres.
Arsenal should have wrapped up the game four minutes into the second half but Pierre-Emerick Aubameyang, with only Alex Meret to beat, placed his shot too near the goalkeeper who was able to save.
The second half petered out after that as Arsenal sat back and Napoli enjoyed most of the possession but did little with it.
Lorenzo Insigne shot weakly at Cech after getting clear of the Arsenal defence and was immediately substituted, a decision which infuriated the forward who watched the rest of the match on the touchline with his arms folded.
Fabian Ruiz summed up their evening by firing their last chance wildly over the crossbar in stoppage time.
Meanwhile, Chelsea reached the Europa League semi-finals by beating Slavia Prague 4-3 helped by a Pedro double in a first-half onslaught before withstanding the Czech side’s comeback bid after the break in a pulsating game.
Chelsea were cruising 4-1 at half-time but Slavia added two goals early in the second period with powerful strikes by Petr Sevcik, which narrowed the aggregate score to 5-3 after the London side had won the first leg 1-0.
Sevcik spurned a chance to cut the deficit further before Chelsea held on to see out the win and set up a semi-final against Eintracht Frankfurt, who won 2-0 against Benfica to go through on away goals after a 4-4 aggregate scoreline.
Chelsea raced into a 3-0 lead within 17 minutes, starting with Pedro chipping goalkeeper Ondrej Kolar.
The Spain winger hit the post four minutes later only for the ball to rebound into the face of Simon Deli and into the net for an own goal.
Pedro carved out Chelsea’s third when he squared for a simple finish by Olivier Giroud and the scurrying Spaniard claimed Chelsea’s fourth goal in the 27th minute when he converted a rebound after Kolar saved from Giroud.
A minute earlier, Slavia’s captain Tomas Soucek had given the boisterous Czech fans something to cheer at Stamford Bridge when he powered in a header from a corner.
A Chelsea triumph in the Europa League, a trophy they won in 2013, would provide a way back into the Champions League if they finish outside Premier League top four this season.
They currently sit fifth in the table.