Diego Simeone takes blame for Atletico Madrid's poor performance after unbeaten run ends
Atletico failed to recover from Jose Maria Gimenez's shock fourth-minute own goal and wasted the few chances they created in a game riddled with errors that the coach blamed on himself.
"I didn't prepare this match well. I am responsible for the plan not working," Simeone told reporters in a press conference.
"Starting with myself first, we have work to do and things to improve. I didn't imagine anything different. The new players need to keep adapting, and I need to get more out of their strengths."
Betis tried ferociously to increase their lead, exposing Atletico's shortcomings, following their 3-1 defeat to Lille in the Champions League.
"Betis are a competitive team who have improved as they showed today, they outplayed us. We clearly couldn't play our game," Simeone said.
"This happened to us against Lille last week, we fell behind in the game and that didn't allow us to level this game.
"Ultimately, as a coach, I have to resolve these situations first and have a clear idea (of tactics) so that the players can perform in the way I like," he added.
Atletico's first defeat of the season leaves them fourth in the Spanish top flight on 20 points, two clear of Betis and 10 behind leaders Barcelona. They face UE Vic in the Copa del Rey on Thursday before hosting Las Palmas three days later.