Debutant Yasir Ali was taken for scans at a local hospital in Chattogram after being stuck on the back of his helmet during Bangladesh's second innings on the fourth morning. Wicketkeeper-batter Nurul Hasan was named as Yasir's concussion substitute, although he won't be allowed to keep wicket as Yasir isn't a keeper.
The incident occurred at the end of the 30th over when Yasir ducked into a Shaheen Shah Afridi bouncer. Yasir briefly took his eye away from the delivery while getting under the ball, and was hit on the helmet.
Bangladesh's physio Bayejidul Islam checked Yasir immediately, and he went back to batting. But an over later, Bayejid came back to check on Yasir during the drinks break, after which he walked off.
The team director Khaled Mahmud confirmed a few minutes later that Yasir was out of the Test match, with Nurul as his replacement. Yasir has been taken to Imperial Hospital for a CT scan. A BCB statement said later that "he is medically stable. However, as a precaution, he will be observed for 24 hours at the hospital."
This is the third time Bangladesh have needed concussion substitutes. The first instance was during the Kolkata Test in 2019 when Liton Das and Nayeem Hasan were struck on the head. Mohammad Saifuddin was also substituted during an ODI against Sri Lanka in May this year.
Mohammad Isam is ESPNcricinfo's Bangladesh correspondent. @isam84