Left-arm fast bowler Riaz injured his bowling arm on the first day of the ongoing second Test in Colombo on Thursday and was subsequently ruled out of the match and the following third and final Test.Sohail had injured his knee ahead of the second Test so was also unavailable to play."Azam and Rahat will replace injured Sohail and Riaz for the Test series against Sri Lanka. Both will leave on Sunday early morning," said a Pakistan Cricket Board (PCB) release.Riaz will return after the second Test match concludes on Monday while Sohail has already returned home.The 26-year-old Rahat, a regular bowler in the longer format for Pakistan since 2013, missed the two Tests in Bangladesh last month after suffering a hamstring injury.Selectors said Rahat has now fully recovered.The 20-year old Azam has yet to feature in a Test but made his one-day debut against Zimbabwe in Lahore last month.The third and final Test will be played in Pallekele from July 3.Frequent injuries in the Pakistan cricket team have been widely criticised by former players and experts, while PCB chairman Shaharyar Khan described the fitness levels in national players as the "worst in the world."As many as five players fell injured before and during the Bangladesh tour last month.Pakistan and Sri Lanka will also play five one-day matches and two Twenty20 internationals after the Tests, the squads for which will be announced next week.