Navas has disappointed Barnes, who feels he is too timid and needs to man up for the tough campaign ahead in England and Europe.
Barnes told the Daily Star Sunday: "I first sat up and took notice of Navas when I was in Malaysia in the summer watching the Confederations Cup.
"Clubs like City and Real Madrid were after him and I thought he must be good.
"He's light on his feet - he is fast. And his performances against Newcastle and Hull were there for all to see.
"But i have been disappointed in him apart from those two games. With his ability he should be ripping defences apart. I'm looking for a lot more from him.
"I was talking to some of the other City old boys like Tommy Booth and we all agree the jury is out.
"Navas is too timid. I hope he has got the strength deep down to show a lot more determination. He's got to show up - and man up."
For breaking stories and all the great banter like us on Facebook: facebook.com/tribalfootball