They did win the Europa League last season, but on the other hand they do look to be on their way out of the Champions League this year.
Sevilla is a decent team. Has had a shot at top four the last handfull of seasons. Personally do not think it will be the size of the club that will draw him away if he does leave. Plus I do not think he would be guaranteed a starting role in the top teams in Europe (already 27 as well). Think it will come down to the amount of playing time and position that Sevilla will hand him. He has started most matches (minus his knee injury) but has only made it the full 90 twice. Sevilla sometimes mixes it up with formation and he has been used as a defensive and attacking midfielder.
It will be interesting to see how their midfield develops over the course of the season. Sevilla lost some midfielders who got game time last season (Denis Suarez, Gerard Deulofeu, and Stephane Mbia) but picked up some new faces to replace them (Steven N'Zonzi, Michael Krohn-Dehli, Evgen Konoplyanka). Currently all three of the new faces have more game time than Banega this season. There is also competition from Iborra, Krychowiak, Vitolo, and Reyes for roles in the midfield. (Though Vitolo, Reyes, and Konoplyanka mainly play on the wings). In fact Banega has the least amount of minutes out of the players I mentioned. (but maybe his injury accounts for some of this? missed 4 games due to knee)
Banega has played with Mori and Deulofeu before. So it is not completely out of the question. Will just depend how things shake out at Sevilla this year and how much we want/need him. It is one to watch.
On a completely different note, my second post! and the league is back this weekend! Stoked.