I don't know about Newcastle's poor performance... It seems to me that on the contrary they showed a good level, considering that this is their first game in the Champions League... Looking at Milan's stats, Newcastle didn't concede a goal and managed to draw away... I think Newcastle played well...
In a game like this most especially in the Champion League group every point is considered vital, Ac Milan created so many countless scoring opportunities from the start of the match to the end. While for Newcastle, it was a heroic point for them, Ac Milan overwhelmed Newcastle it was a disappointing result to me for Milan having had a lot of opportunities to have won the match but could not find a way through Newcastle played like they were underdog and in many ways there were not they were outplayed but not outmatched
Champions League is completely different world with every action and point is going to count at the end which is important for any team here we have one of the boring start of the AC Milan and Newcastle United with advantage is clearly going into favor of the Newcastle United because they were here after two decades and their coach is also relatively new with no experience but still they have done good job and having one important point for AC Milan this was really embarrassing with too many attacks and scoring shots but at the end they were fail to produce anything positive for themselves specially after one of the worst defeat against Inter Milan things were into their favor for the win, but they fail which is surely going to have problems for them in coming matches.