I didn't drop Sissoko or Borris when they were being targeted even though their morale was through the floor, and they still performed perfectly well (perhaps not brilliantly, but good enough). What happened was other players 'stepped up to the plate', first Borris which made him the target of opposition managers and of course Nicoise.
The bug seems to be affecting morale, not performance, or at least it is for me... None of the players that have been targeted by AI managers have had really bad games because of it (or not that I could tell). Poor performances have, generally, been a team problem in the games we've lost, rather than any single player.
It's an annoyance when the rest of the team has good, v.good and superb morale and one of your key players has v.poor, sure, but if it's not affecting the player's or the team's performances, there isn't really a problem... is there?
