Keita saves Pep from season-ending defeat
After three consecutive league titles, Barca finished the campaign second behind champions Real Madrid with 91 points from 38 matches and a game in which neither side had anything left to play for.
Guardiola has one fixture remaining, the King's Cup final against Athletic Bilbao, before handing over the reins to his friend and assistant Tito Vilanova.
The Catalans will be chasing their 14th trophy from the coach's four years in charge at the Calderon on May 25.
"I don't think the league will miss me. The league was here before, and will continue after me," a downbeat Guardiola told a news conference.
"After Dani's sending off it was a different game. We lost control because playing with 10 is always more difficult. In the end it was a good goal from Seydou.
"He is a player who probably deserves to play more."
Earlier, Valencia, 30 points behind Barca in third spot, said farewell to coach Unai Emery after a 1-0 defeat at Real Sociedad.
French winger Antoine Griezmann scored the only goal after 64 minutes.
Argentine Mauricio Pellegrino will take over from Emery who is reported by local media to be close to joining Spartak Moscow.
Emery, a manic touchline presence, leaves Valencia after guiding them to automatic qualification to the Champions League for the third year in a row.
Much of the pre-match attention for the Barca game had been on whether Lionel Messi would add to his record 50 league goals this season, but the World Player of the Year had a quiet game and was upstaged by Betis striker Ruben Castro.
Lively Dutch international Ibrahim Afellay started his first league game for Barca this season, after a long injury lay-off, and Sergio Busquets headed them in front from a corner after nine minutes.
They lost control of the game when Brazil full-back Alves picked up a second yellow card for a scything late tackle on Jefferson Montero in the 53rd minute.
Castro burst clear to lob goalkeeper Victor Valdes in the 71st minute and he also broke through the middle, after a swift interchange of passes, to add his second goal three minutes later.
Messi struck the post with a free-kick just before Mali midfielder Keita flung himself at a cross to level matters.