It's unfortunate he used the gesture, yes. Raed you said he deserved every bit of it, but there are better ways to deal with what he did. In fact a speech in front of the whole squad would not only teach him a lesson but also humble him. This way both coach and player will have consequences, I just hope that this incident won't influence the future of Ljajić in a bad way.
I'm slowly starting to believe that Seria A is not good place for youngsters to develop at all. I was happy for him to hear he joined Fiorentina, not only cause it's a beautiful place to nurture his talents but also cause Jovetić and Seferović were there. I still don't get why Seferović is loaned, he just turned 20. At first I was happy Ljajić didn't signed for United, but perhaps that deal would be better than the situation he is in now (not necessarily the playing time he gets, but the nature of Seria A in general).
This coach was lucky Ljajić isn't like the average player ending with 'ić', if Ljajić was, both player and coach (with a small possibility that the player would face jail time) would probably end up with long suspensions.