Mohammad Isam is ESPNcricinfo's Bangladesh correspondent. @isam84
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"Having recovered from his previous groin injury ahead of the first Test and successfully managing to bat in the Bangladesh first innings as well as send down six overs, all-rounder Shakib Al Hasan has suffered a strain in a different region of his left thigh while fielding on day two," a BCB statement said. "The injury was confirmed by an MRI scan this morning.
"The BCB medical team will continue to treat and review his progress over the course of the first Test."
Eyebrows were raised when Shakib didn't take the field on Friday morning, mainly because of the injury earlier in the series. In the ongoing Test, he appeared in pain when trying to stop a ball with his foot off his own bowling in his third over on the second afternoon.
He went on to bowl a few more overs but after completing his sixth, Shakib slowly walked off the field when Shadman Islam's foot injury gave everyone a break.
This is Shakib's comeback Test match after his one-year ban by the ICC ended on October 29 last year. He made a successful return in the ODIs, winning the Player of the Series award, but it was somewhat marred by the groin niggle - at the time, he had said that "it [the injury] isn't looking good".
Meanwhile, Shadman has returned to the field after being hit on the foot during the second day's play.
Updated following the BCB statement at 8.40am GMT on the third day of the Chattogram Test