Celtic paid a fee of £5m to sign the Greece international from AEK Athens in the summer to fight with Scott Bain for the number 1 position at Parkhead.
But Barkas has looked shaky all season and conceded four more goals on Thursday night when Celtic lost 4-2 to AC Milan in a Europa League game at the San Siro.
Celtic’s defending has been poor this season and the lack of a commanding number 1 has for many only compounded their woes at the back.
McManus laid into the Greek goalkeeper and conceded that he has rarely seen Barkas make a save this season, labelling him a human polo mint because every shot seems to go through him.
He believes Barkas has proved that he is just not good enough to stand between the sticks for Celtic.
McManus said on PLZ Soccer: “I think Barkas so far, I have yet to see him make a save.
“Everything goes through him, even the goal in the Old Firm game, the header, goes through him.
“The third goal last night, it was a great bit of play from the AC Milan winger who cuts inside, but again you can save that, parry that, get something on it.
“Everything goes through him.
“He is like a human polo mint.
“For me, he is a bit of a busted flush at the minute and they’ve got to look to try and get someone else in.”
Celtic will not have long to wait to have the opportunity to sign another goalkeeper as the transfer window swings open in less than a month.
