Too complacent for me. Like us, they didn't get tight enough on the Juventus runners and ultimately paid the price.
They lack experience in Europe and that counts for a lot. They should have closed the game down and reacted to Juve's substitutions. Pochettino afterwards was saying it was a good result but they should never have lost from that position. They'll learn with experience to play European games differently to domestic games.
They're becoming a nearly team. So much ability and promise but so far there is no silverware to show for it. I can see some of their star players moving on if they don't start winning things soon.