Celta Vigo is very lucky that Barcelona did not play with a winning mission in the last match which allowed them to win that kept them away from the relegation zone. The defeat in the last game did not have a big impact on Barcelona even though their winning record was tarnished, Barcelona played only to meet the requirements as a sign that the season was over. An additional three points allowed Celta Vigo to stay in La Liga next season. Real Valladolid, Espanyol and Elche were relegated to Laliga Smartbank.
I don't even think if celta vigo was lucky in this case. in my opinion that if it barcelona was giving up on purpose against celta vigo. Barcelona has intentionally given up since the match was starting.
No reason for barcelona to win against another club after it was winning la liga trophy at this moment. In my opinion if this is caused by barcelona is willing to help another club to get full points. The season was almost ended at that time and barcelona knew how important three points for other club. that's why barcelona was letting other club to beat it include celta vigo as well.
Celta vigo was lucky to get full points from barcelona. that luck has been given by barcelona. Barcelona was always losing full points once it has been winning the trophy.
It seems like that was the main motive owned by barcelona at this moment.