The Spaniard was talking to Sky Sports ahead of the Gunners' clash with their London rivals at Stamford Bridge.
Arsene Wenger's side have begun turning things around by beating both Bournemouth and Cologne but Merson thinks Arsenal will capitulate like they did against Liverpool if Chelsea score first.
A lack of supply from his teammates could be highlighted by Lacazette's fewer shots, although he has been more clinical than Morata, converting 30 per cent of his chances compared to 21 per cent from the Chelsea striker. They have so much guile now Eden Hazard is back, a good system and players who can hurt you as well as those who can defend. They are one player short of being virtually unstoppable.
Hazard cut inside and fired at Cech at the other end, but a disappointing second half did have a dramatic sting in the tail when David Luiz lost possession and jumped in high on Kolasinac as the Blues ended another match against Arsenal at a numerical disadvantage, the Brazil international receiving his first-ever Premier League red card. But his knee knock was probably still affecting him as the ball got a bit stuck under his feet and all he managed was a tame chip that deflected easily off Čech's shoulder. It's like when you watch something and someone's done something in 15 minutes and you think 'wow, they can play' and he done that about five times.
"I like Lacazette don't get me wrong, but if I was playing Real Madrid on Saturday he would play in front of Lacazette".
Arsenal spent £46 million landing Lacazette from Lyon, while Chelsea paid Real £60m for Morata.