The rationale behind the proposal is well-intentioned. At present, if an injured player must leave the pitch, their team is temporarily at a numerical disadvantage. Hence, there may be an incentive to committing a foul that requires an opposing player to receive off-field attention. Any policy that eliminates this incentive is to be welcomed.
Similarly, the ‘injured’ player may not have an incentive to get back to the pitch as soon as possible in order to keep the opponent off the pitch (in the case of genuine injuries, this would be welcome as the need to get back on the pitch to help the team is reduced).
Where games are particularly close or important, especially near the end, this incentive outlined above may be greater. As with any form of ‘gamesmanship’ or cynical play, facilities like retrospective action, video evidence, independent medical assessment and severe punishments can be utilised to prevent such behaviour.