Mohammad Isam is ESPNcricinfo's Bangladesh correspondent. @isam84
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Bangladesh had to use two concussion substitutes after Liton Das and Nayeem Hasan were both hit on the helmet by Mohammed Shami. The visitors' choice of replacements, though, put their planning for this series in poor light, as they had no specialist batsmen in reserve despite two batsmen in the original squad going unavailable well before the start of this match.
While there was one bowler-for-bowler swap with Taijul Islam taking Nayeem's place, Mehidy Hasan, a specialist spinner, took over from Liton, a wicketkeeper-batsman. That means he cannot bowl in the game. As per the ICC playing conditions, a concussion substitute has to be a like-for-like replacement and perform the same role as that of the concussed player.
Both Liton and Nayeem went to the hospital for scans, where concussions were confirmed. "Both players have been diagnosed with delayed onset of concussion by the medical team after being struck on the helmet while batting," a BCB statement said. "Consequently their symptoms have ruled them out of the rest of the Test Match and they have been substituted with Mehidy Hassan Miraz and Taijul Islam respectively. They will now follow their concussion recovery protocol to ensure their safe recovery."*
Saif's injury was confirmed on November 20, two days before the game, and although there are 30-minute flights between Dhaka and Kolkata, no one was was flown in. Mosaddek, meanwhile, had left the Test squad on November 11, even before the first Test, to be with an ailing family member, but the BCB didn't name a replacement then either. This, in a squad missing Shakib Al Hasan and Tamim Iqbal.
Liton was hit on the helmet by a Shami bouncer in the 20th over of Bangladesh's innings. He decided to bat on, struck two more fours, but walked off seven balls after the injury. Nayeem was hit off the third ball he faced and he too continued batting, whacking Shami for four boundaries before he lost his off stump to Ishant Sharma after making 19 runs.
The concussion-sub rule came into effect in time for the Ashes earlier this year and since then it has been used five times in men's Test cricket and India have been involved in four of them.
In August, Steven Smith was replaced by Marnus Labuschagne after being hit by Jofra Archer. A few days later, in the Caribbean, Darren Bravo took a blow from Jasprit Bumrah and Jermaine Blackwood took his place. In October, Dean Elgar found himself in trouble after being struck by Umesh Yadav, which brought Theunis de Bruyn into the game. There is potentially a sixth instance on the cards with New Zealand's Tom Blundell being on standby for Henry Nicholls, currently playing a Test against England.
*This copy was updated at 5pm GMT with the Bangladesh Cricket Board's official statement on the concussions.