An interesting one.

If it was the tackle that caused the injury (as opposed to the player simply twisting his ankle) the red card was probably appropriate.

However the referee didn't even think it was a free kick let alone a red card. If it hadn't been for Salah's intervention the game would simply have continued.

The referee also didn't seem to go to a television monitor to view a replay of the tackle and form his own opinion.

If the player hadn't been injured would the VAR observer still have instructed the referee to show him a red card?