I found the clip on the 'asktheref' site.
Below is the answer that one of their referees gave.
i queried their answer for the same reason that jpm has given above, i.e. as per interpretation of law 3, 'player outside the field of play', pg 65 of current lotg handbook plus their incorrect understanding of Dogso
Their reply was also incorrect in that the player
can enter the fop over the goal line when play is stopped, with the ref's permission, of course - tho' i let that one go
The idfk, by the way, is for stopping play to caution the player for entering the pitch without the referee's permission - to be taken from the position of the ball when play was stopped
'As the player was off the
field of play receiving treatment he must wait for the referee's permission to re-enter the
field of play and can only do so at a touchline.
In this scenario the player is cautioned for re-entering the
field of play without permission and the restart is an indirect free kick from where the
ball was when play was stopped which in this case will be on the 6 yard line as it happened inside the goal area.
By the way the player cannot be sent off for his action. He would have to do something more such as deliberately handling of the
ball (DOGSO H), commit a foul that meets the four criteria for denying an obvious goal scoring opportunity (DOGSO F) or commit a foul requiring a 2nd caution. Kicking or heading the
ball in play does not merit a caution.
Also prevention is better than cure. It is always a good idea to move the injured player to the touchline in these situations rather beside the goal, the player is going to have to come on from the touchline anyway. While we don't do it perhaps forcefully enough it is also a good idea to remind players to wait for the signal to return.
An interesting aside here is that once the player stepped back on to the
field of play he had to be taken into account in determining offside even though his action was illegal. It is assumed as the physio was present that the player was removed for treatment by permission.'