The 38-year-old will be out of contract in the summer and has said that he would like to stay at Rangers beyond the end of his current deal.
The veteran striker has been a bit-part player at Rangers this season and has also struggled with injuries.
McManus conceded that his ageing body has felt the effect of wear and tear this season, but believes a fit Defoe still has a lot of offer to Rangers.
The former Hibernian man insisted that if he stays that the veteran striker can offer his experience to the youngsters at Rangers and feels there is a lot to learn from him.
McManus also stressed that he still has something to offer on the pitch as a substitute from the bench.
The former attacker said on PLZ Soccer: “He has obviously come to the end of his career.
“I think this season he has also picked up a few more injuries than usual.
“I think that is just part of getting old, his body is feeling him a little bit in terms of day to day training.
“I still think he has got a part to play.
“When you look at Rangers possibly losing [Alfredo] Morelos in the summer.
“If you can get a fully fit Jermain Defoe, he can maybe come on for 15 to 20 minutes in tight games and he can pass his knowledge down to the B team at Rangers and the Under-18s.
“He can go and maybe do finishing drills with them.
“You can see how much of a professional he is and how much he looks after himself off the pitch to still play in the level he is playing at 38-39 years old.
“I think it’s a no-brainer for Rangers to try and keep him in some sort of role because he is a fantastic pro and still got a bit to offer on the pitch as well.”
Defoe has four goals in 14 appearances for Rangers this season in all competitions.
