"Sami is a mentally weak athlete and he can't take pressure," Akhtar said.
Akhtar turns his attention to Sami, a few days after criticising the Pakistan Cricket Board.
"Efforts were made to make Mohammad Sami into the 'Karachi Express'. He was constantly told that your competition is with Shoaib Akhtar for a place in the team," Akhtar said.
Akhtar, who retired from all forms of cricket in acrimonious circumstances during the World Cup last year, was asked about the performance of Sami and his experiences of playing alongside him.
Akhtar insisted that when he played his aim was always to try and get the opposition out in one spell.
"I was always aggressive and trying to get wickets as that is the job of a fast bowler like me," he said.
The national selectors and the team management have come under criticism for retaining Sami in the T20 squad announced for the coming Australia series and the T20 World Cup, after he failed to make an impact on the Sri Lanka tour.
Sami's selection was a debatable one, with critics pointing to his abysmal Test record of 85 wickets in 36 matches at an average of 56.
His performances in ODIs and T20 matches are better as he has captured 121 wickets in 85 matches at an average of 28 and 10 wickets at 14 in five T20 games.
Pakistan's former captain, Waqar Younis, also had recently criticised the performance of Sami in Sri Lanka, urging the selectors to go for young blood and to repose more confidence in someone like Junaid Khan.
The selectors didn't pick Junaid in the T20 squad with chief selector, Iqbal Qasim, making it clear that captain Mohammad Hafeez and coach, Dav Whatmore, had preferred to have Sami instead of the youngstar.
"Sami has failed to learn from his past experiences and despite making numerous comebacks, he is still a very expensive bowler," Waqar had said.
Akhtar said that he always tried to bowl as fast as he could even if he had to bowl just 12 overs in a day.
The Rawalpindi Express observed that in a bid to compete with him, Sami also tried to bowl fast all the time but couldn't control his line and length.
"I played a lot with Sami and my observation is he tried to compete hard and lost discipline in bowling. When we bowled together we could put a lot of pressure on the batsmen and in such circumstances he bowled well," Akhtar said.
The former pacer believed that luck also plays a big role in any cricketer's career.
"Sami was an unlucky bowler as well many times straight forward catches were dropped off his bowling while in some instances umpires didn't give simple decisions in his favor," he added.
But Akhtar believed that Sami, compared to him, Wasim Akram or Waqar Younis, was a mentally weak bowler.
"He has been playing for 11 years but if you tick him off once he loses focus and comes under pressure. He has the ability and I wish he does well now but for this he has to be assigned a role in the team and pressure must be put on him to perform as a frontline strike bowler," Akhtar said.
He noted that the Pakistan team had a lot of potential and should go onto the field with the belief that it could bowl out any opposition in one session.
"But when you look at the present side you can see they go onto the field convinced that they will have to field for at least three to four sessions."