There's no precedent to look to to guess what the sanction may be.
It could be a small fine or it could be expulsion from the play offs.
If it had been in the league, I guess there’d be a fine and a 3-0 forfeit of the game, but that can’t really be done now having played out the second leg at 0-0.
My gut suggests they’ll get a hefty fine and if they go up, sued by Middlesbrough for potential loss of earnings. All depends on the evidence you mentioned. If it’s multiple times, it could be expulsion.