Robson spent five years at the club as a player and seven years as a summariser for Arsenal TV before leaving last summer.
He told Suedeutsche Zeitung: "The debate is always about him spending money. But it would be better to analyse why the team is defending abysmally and have no balance.
"Everything is off the cuff. There is no tactical plan. I see the same mistakes at the back.
"Why are the full-backs always in the wrong positions?
"The fans are screaming for better players but you just have to coach the ones that are here properly.
Robson added: "Of course he wants to win the Champions League.
"But did he have to rest half the team against Blackburn? You can maybe do that in the league, where you can make up for a bad result. But not in a cup competition."