Thing is, Udinese are still level on points at the top of the group with Atlético and have a pretty good chance of going through even with a weakened team. I suppose they want to put in minimal effort now and if they're rewarded later on, then that's a bonus as it hasn't had much effect on their Serie A campaign. They actually gave Arsenal a decent couple of fixtures, but were just worse in front of goal and had chances hit the post (+ missed penalty). If only they realised that a semi final/final was a big possibility if they tried, it might motivate them.
I didn't watch tonight's game, but they already beat Atletico with Benatia and Pinzi being two changes to the team, so I don't know if they were having an off night or Atlético just seemed to care more.