Celtic got back to winning ways on Sunday when they beat Motherwell away from home in a league game and are now currently nine points behind Rangers in the table having play two games fewer.
While the scoreline looked comfortable, Motherwell did score in the second half to make it 2-1 before the Bhoys scored two more to seal the three points in their favour.
McManus stressed that compared to Rangers, Celtic do not look defensively secured and always look like conceding goals at the moment.
The former Hibernian man pointed out that Rangers are not even conceding chances and are keeping clean sheets for fun, but Celtic do not look like achieving a clean sheet.
McManus said on PLZ Soccer: “I still don’t think Celtic’s defence are quite at it at the minute.
“You look at Rangers at the minute, they are not conceding any goals or conceding any chances.
“I feel Celtic got the goals at half time and in the first 20 minutes of the second half, they tried to coast it.
“They tried to coast and see the game out against Motherwell and Motherwell came into the game.
“They obviously got a rocket at half time from the manager, but I think Celtic came out and were very sloppy and very relaxed and the Motherwell goal was coming, you could sense it.
“When the Motherwell goal went in, Celtic stepped up a gear and got the killer third goal.
“But I just don’t think Celtic’s back three or back four, whatever they are playing, I just don’t feel they are really secure.
“Especially when you look at the other team across Glasgow who are keeping clean sheets for fun.
“Celtic don’t look like a team who are going to keep a clean sheet at the minute.”
Rangers have kept eight clean sheets on the trot in all competitions since the start of October and Neil Lennon will look for Celtic to become more solid at the back soon.