Emery replaced Arsene Wenger at the end of last season after the Frenchman spent almost 22 years at Arsenal.
The new boss brought five players into the transfer window and ran a number from the exit door.
Defenders Stephan Lichtsteiner and Sokratis Papastathopoulos, midfielders Lucas Torreira and Matteo Guendouzi and goalkeeper Bernd Leno form the quintet.
Merson says that Emery has signed up for the type of players he needed, but has doubts whether Lichtsteiner will be a regular starter at 34.
"He bought players who needed them," Merson said on Sky Sports.
"He needed defenders, he gained experience in Lichtsteiner.
"But, 35 years old, how many 35-year-olds play in the Premier League, especially on the back.
"But he's got experience, it's going to be different, he needs time."
Arsenal start tomorrow in the Premier League at home against outliers Manchester City.
Next weekend, they meet in Maurizio Sarris first home game on Chelsea.
And Merson says he would not be surprised if Arsenal were in the relegation zone after the opening games.
"He could not have wished for a worse start, Man City at home, Chelsea away," he added.
"They could be in the bottom three positions after two games, which would be a shame for the coach.
"Tomorrow, you'll see a team making Man City very hard to break.
"It will not be like in the old days, where we have a shot, you have a shot."