The Gunners have lost their opening two group games and now face a double-header with Bayern Munich.
It was a similar story in the second half with Arsenal equalising on 65 minutes from a well-taken Alexis Sanchez goal before conceding a minute later on the counter-attack.
Conceding so soon after both equalisers was further evidence of the tactical naivety Arsene Wenger’s men have displayed in the Champions League in recent years and Keane was unflinching in his assessment.
"Listen, they're soft. Arsenal are soft. They're weak. The goals they give away… you've got no chance of winning any big game if you defend like that,” he said on ITV’s Champions League highlights show.
"I think there's certainly a weakness to the group, they look like they lack characters, leaders and winners.
"I enjoy watching Arsenal, I think they're great to watch, particularly going forward. But part of football is, unfortunately, is defending. And mentally, they're weak,” he added.
Having a soft centre is a claim that has been levelled at Arsenal since the losses of the likes of Patrick Viera and Gilberto Silva and it’s an issue Wenger has refused to address in recent transfer windows.
Coquelin has provided some much-needed bite in midfield but he is by no means the finished product and partnering him with converted attacking midfielders like Santi Cazorla and Alex Oxlade-Chamberlain is a risky tactic that all too often doesn’t pay off for Arsenal.