Ravichandran Ashwin, one of India's best ever, who completed 500 wickets in Test cricket on Friday, February 16 against England in the ongoing Test in Rajkot will take no further part in the game due to a family emergency. India will have to make do for the rest of the match with an Ashwin-sized hole in the side, especially when England have gotten off to a rollicking start in their first innings with the bat.
As Ashwin withdrew, there were several doubts and questions about whether India could seek a replacement for the senior off-spinner in the team for the remainder of the Rajkot Test. And the answer is no. As per the laws, Team India will be allowed a substitute fielder for the rest of the game in Ashwin's place as per the MCC Law 24.1.2 but not a replacement.
The replacement as per the existing rules can only be allowed by the officials in case of concussion or COVID-19 (if it's still the case since many players have featured in the games despite testing positive).
There's a provision in the law that allows a captain to replace a player already named in the side if the opposition captain allows it. As per 1.2.2, "No player may be replaced after the nomination without the consent of the opposing captain." However, that provision comes under consideration from after the toss till the start of the match. Hence, India could replace Ashwin only if a player was nominated before the toss for a scenario like this with the consent of England captain Ben Stokes, which seems unlikely now.
Once the game has begun, only a concussion or a COVID-19 case will be eligible for replacement.
It will be a significant blow to the Indian team as the bowling attack has looked like a pale shadow of itself against England's belligerent hitting and they will hope for some individual brilliance to get their side back into the contest. England are still 238 runs behind and India will have to be on the money from the get-go.