Newcastle actually seems to be playing worse in the domestic league than AC Milan, who only lost once against Inter Milan yesterday. Ac Milan performed well at the start of this season while Newcastle has not shown good results in the EPL league, experiencing three consecutive defeats even though last week they narrowly won. against Brentford, but I think these two clubs definitely want to win, especially playing at Acmilan's home ground, of course Newcastle will find it difficult to win this match too.
Generally AC Milan have better net result than Newcastle United but recently, last weekend they both have bad performances and results. In Champions League, AC Milan is a bigger club and in the last season, they went too far. Newcastle United is like a newbie in Champions League and they will face with one of most successful clubs there, AC Milan.
But no one knows the strength of the two clubs in the domestic league and European championship league like the UCL, sometimes it always brings surprises that we don't imagine, playing bad in the domestic league doesn't necessarily mean playing bad in the UCL, I think the result of this first meeting will end in a draw and have to share points.
They are different competitions and players would have more motivation to play well in Champions League. Because it is barely chance to play in Champions League. Not all clubs can participate in Champions League regularly so playing there is already a dream of many players.
It takes decades for Newcastle United to join Champions League again. In Premier League, Tottenham Hotspurs is another example. They had one Champions League season ago but recent seasons, they struggled to get a position in top four of Premier League. So they less appear in Champions League recent years and players like Kane, Son Heung Min would feel unhappy with it.