Hart started between the sticks for Celtic in their Scottish Premiership opener against Ross County on Saturday afternoon.
The Bhoys eased to a comfortable 4-2 win, but McManus thinks that it is clear the defence are nervous about having Hart behind them.
He believes that against better quality opponents, Celtic would have been undone and replacing Hart has become a priority for the Bhoys given where they want to get to over the course of the coming campaign.
McManus wrote on Twitter: “I remember saying Celtic had to upgrade Joe Hart way back into last season and was laughed out of town.
“Not good enough for where Celtic want to go.
“I’d say that area is now a priority.
“Nervousness evident in that Celtic backline against better opponents would have been punished.”
The transfer window remains open for another month, but the jury is out on whether Celtic boss Brendan Rodgers is preparing to bring in a new goalkeeper.
Celtic have Champions League group stage football on the agenda, as well as a domestic treble to defend this term.
