The forward spun away from his marker and fired low into the corner nine minutes from time, sparking a desperate final charge from Celta.
Betis keeper Adrian had to block a header on the line in the closing minutes and Mario Bermejo finally worked his way round the visiting custodian, only to fire his shot against the post with the goal gaping in time added on.
The Andalusians are fifth with 28 points, behind Malaga in fourth on goal difference, and a massive 18 points adrift of runaway leaders Barcelona.
Unbeaten Barca crushed second-placed Atletico Madrid 4-1 with a double from Lionel Messi at the Nou Camp on Sunday, moving nine points clear at the top, and 13 ahead of champions Real Madrid who were held 2-2 at home by lowly Espanyol.
In Monday's other game, Deportivo La Coruna remained at the foot of the standings on 12 points after a 0-0 home draw with Valladolid.
(Reporting by Mark Elkington, editing by xxxxxxxxx)