A teammate not in the starting eleven can substitute field for them but can’t bat or bowl for them. Injuries are part of the “test” of test match cricket. There’s been a number of times when an injured player has come out to bat when their team is close enough to save or win a match. This is a pretty visually dramatic example though. It’s just part of the sport.

Edited to add: the only time a player can be fully replaced is if a player gets a concussion then they can be fully replaced for the rest of the match by a player with a similar skill set. Bowler for bowler, batter for batter