Duffy joining the Scottish champions in the transfer window was seen as a coup for Celtic, who convinced the player to move away from the Premier League.
But the 28-year-old has been struggling since the start of the season and was again poor in Celtic’s 4-1 defeat at Parkhead to Sparta Prague on Thursday night.
Questions have been asked about why Duffy has struggled in Scotland, but McManus feels it is down to the fact that he played a different style of football at Brighton compared to Celtic.
He insisted that Brighton played more defensively and Duffy was offered protection which is completely different at Celtic, where players are bombing forward and the defender is as a result being exposed.
McManus also stressed that Duffy is not great when trying to defend one on one and has struggled to cope at Celtic.
He said on PLZ Soccer: “I think Shane Duffy obviously used to playing in a different style at Brighton.
“Brighton were backs-to-the-wall for most of the game, he had plenty of protection in front of him in midfield and the two full-backs tucking in.
“I think at Celtic, particularly at home, they just bomb everybody forward and they don’t sit back.
“Even in Europe at home, last night you have seen that, two full-backs bombing forward and he is getting left exposed.
“And I don’t think he is a particularly great one-v-one defender.
“I think it is a totally different style, the same with Ireland away from home from backs to the wall and it’s a totally different style.”
It remains to be seen whether Duffy gets another start when Celtic travel to Motherwell on Sunday.
