The Gunners were active in the transfer window bringing Pierre-Emerick Aubameyang in and signing Henrik Mkhitaryan in the Alexis Sanchez deal.
Arsenal they failed to sign a defender after going for Jonny Evans on deadline day.
And Merson reckons their inability to sign a new defender will leave them short until the summer.
“Arsenal will be Arsenal,” he told Sky Sports.
“One week we’ll be lavishing them with praise for their attacking football but there will always be results like Bournemouth and Swansea around the corner.
“We’ve never questioned them going forward but in this window they’ve improved their attack with the acquisitions of Henrikh Mkhitaryan and Pierre-Emerick Aubameyang, but they haven’t improved at the back and that will be their problem until the end of the season.”
And Merson thinks they will struggle to break back into the top four.
They sit sixth and are eight points behind Chelsea and Liverpool.
“It’s tough but I’m going to go with Arsenal and Chelsea missing out,” he said.
“For me, Tottenham can give anyone, including Man City a game, on their day and results like the one they picked up against Manchester United will only build confidence.
“Lucas Moura is also an excellent signing from PSG and he will make them even stronger as we get to the business part of the season.”