Iyer hasn't been picked in the Indian squad for the last three Tests against England.
Iyer failed to cross the 50-run-mark in his last 13 Test innings and he looked unsettled while batting in the first two Tests against England, registering scores of 27, 29, 35 and 13.
Speaking in a video by ESPNCricinfo, Manjrekar advised Iyer to work on his defensive game if he wants to excel in Test cricket.
"Iyer has to decide which are the formats he is going to try and excel in. If Tests continue to be his priority, then he has got to work on his defensive game whether it's pace and bounce or spin. Develop a game where he is confident in defence and then when he takes the aggressive route it is an extension of the defensive play where he is trying to dominate and not escape pressure by playing a counter-attacking game," said Manjrekar.
Sharing his predictions for the third Test line-up, Manjrekar said KL Rahul will return to the Indian XI while adding that he has no idea about how the team management is thinking about the wicketkeeper spot.
KS Bharat is yet to make a mark with bat in his seven Tests so far, scoring 221 runs in 12 innings at an average of 20.09 without a single half-century.
Besides Bharat, Dhruval Jurel is the other gloveman in the Indian squad announced for the last three Tests against England.
"Rahul definitely comes in for a batter. Have no idea what the team management and the selectors are thinking about the keeper. Before Rishabh Pant comes and takes his rightful place in the Test squad, would like to have a look at some of the other options India have," Manjrekar added.
Sanjay Manjrekar also opined that with pacer Mohammed Siraj around and reverse swing giving quicks some help, India could stick with two seam bowlers and a finger spinner in the third Test.
"If Mohammed Siraj was not available as a second seamer, then it would have been easier for India to play four spinners and go the English way. But with Siraj available, and with the reverse swinging creating opportunities for wickets for the seamers, and with the way England have played the finger spinners, India might stick to two seam bowlers and a finger spinner making way," said Manjrekar.
After the first two Tests, the India versus England series is evenly poised at 1-1. While the Three Lions won the series opener in Hyderabad by 28 runs, the hosts bounced back to win the second Test in Vizag by 106 runs.
The third Test is slated to get underway on February 15 in Rajkot.
India squad for last three Tests: Rohit Sharma (C), Jasprit Bumrah (VC), Yashasvi Jaiswal, Shubman Gill, KL Rahul, Rajat Patidar, Sarfaraz Khan, Dhruv Jurel (WK), KS Bharat (WK), R Ashwin, Ravindra Jadeja, Akshar Patel, Washington Sundar, Kuldeep Yadav, Mohammed Siraj, Mukesh Kumar, Akash Deep