Given the odds for unders/overs in Osasuna v Sevilla, Galilee, I'd be more inclined to go for the overs, it's certainly much better value, and I'd say it stands a reasonable chance of coming in.
Sevilla's last 4 games in Europe have been overs (2-1 and 2-2 v Spurs, 2-2 and 2-2 v Shakhtar). They've also scored at least 2 goals in all their away games in this competition except for the very first one, v Liberec.
They may well look to grab an early goal (like they did away to Spurs in the
UEFA and away to Deportivo in the Copa del Rey), if that happens Osasuna would be forced to go in search of an equaliser, leaving Sevilla free to play on the counter-attack, and they can be lethal in that kind of situation, as Deportivo found out in the Copa del Rey (it finished up 3-0 to Sevilla).
Having said that, Osasuna's defence has been pretty solid in the knock-out phase of the
UEFA cup, only 1 goal conceded in 6 games, and they've only conceded one goal at home in the competition (and that was on own goal, v OB in the group stages).
If you asked me to choose, I'd just about edge towards unders, but as you say, at the price, there's no value in that.