Sergio is a multi-millionaire man Prigioni... he was too young when he went to the NBA, and had the terrible luck to go to a McMillan coached team. But now he's maturing in Real Madrid, playing good to great D and getting ovations from the opponents fans (see at the end). He's the best PG in Europe right now, period.
I don't think he's going anywhere until he wins a Euroleague title or two with his current team. That's also true for Mirotic (Bulls) or Llull (Rockets), who will be better if they go to the NBA with a Euroleague title under their belts. That's what Manu did, or what Rubio (even at his young age) did. They also need to improve their defensive game.
The Celtics could use Sergio if he's willing to come back to the NBA. He's averaging 12.4 ppg and 6.4 apg in 21 mpg in the ACB
(link) and 11.6 ppg, 5.6 apg and 1.9 spg in 19 mpg in Euroleague
(link). The assist numbers are great, because in FIBA assists are harder to get.
He's probably at the top of his game, but he's a much better player than in his NBA days. I'd like him on the Cs to backup Rondo. He can also play the 2, as his shooting is great. But I'm loving what Sergio "El Chacho" Rodríguez is doing with Real Madrid, they're the most entertaining team in 20 years in European Basketball, and so far they're 14-0 in ACB and Euroleague combined. If he's not wearing Celtic green, I want him to stay with Real Madrid.
Here are the highlights of their last ACB match, around the 1:20 mark, you can see a great play by el Chacho, and hear Sevilla (they played on the road) fans saying "woooow":
http://www.rtve.es/alacarta/videos/baloncesto/cajasol-62-real-madrid-97/2184887/