Frimpong is concerned for the Gunners, who came from behind twice to beat Aston Villa with 10 men last weekend but contrived to blow a two-goal advantage over Watford the previous week.
Their ex-midfielder, who turned out 16 times for the north Londoners from 2011-14, believes the current landscape is bleak.
"Our defence is certainly weaker, when you compare it to Liverpool or Man City. David Luiz has been making some mistakes, and in my opinion there’s little difference between Luiz and Mustafi; I would rather see Callum Chambers selected.
"They need to be buying better defenders, like [Kalidou] Koulibaly, who can make a big impact on Arsenal’s team."
The Ghanaian midfielder, who rose through the Gunners' academy ranks, is unsure whether Unai Emery is the right man to take the club to the next level.
"I don’t know if Emery is or isn’t the right man to take Arsenal forward," he said.
"All I know is that Arsenal players need someone to really motivate them because you can see at Liverpool, every game they are always motivated, they press every team.
"With Arsenal I feel like players are too comfortable. They have bad games and they are still selected.
"I think that he needs top four; we need to get into the Champions League this year. If we get into the Champions League this year, I think he has done well."
Arsenal face Manchester United at Old Trafford on Monday night, and Frimpong believes the clash isn't what it used to be.
He says the increased competitiveness of the Premier League demands that each club raises the bar by buying top players.
"The Premier League is getting harder every year, because better players are coming in and making it tougher," he said.
"The Premier League has come a long way in the last 10 years, so we just have to get better players. I believe Pep Guardiola is a top manager, but without the players that he has got, he is nobody. You can’t do anything.
"Arsenal need to buy better players – it’s very simple."
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