Celta were leading 1-0 at the Madrigal and there were about three minutes left when the canister, which local media said contained tear gas, landed near the visitors' goal and began spewing out a thick cloud of white smoke.
The players, some of whom were clearly suffering from the effects of the gas, retreated inside the stadium and fans quickly left the ground, with many covering their mouths and noses and in obvious distress.
The match was eventually restarted in an almost empty arena and Celta forward Nolito made it 2-0 from a free kick in the 90th minute.
The result meant Villarreal missed a chance to close to within a point of fourth-placed Athletic Bilbao.
- Sports & Recreation