A Ruben Castro brace spoiled a fine Valencia debut for Barcelona loanee Munir El Haddadi.
The striker nearly scored right after coming on in the 63rd minute, firing a 20-yard strike against the crossbar with goalkeeper Antonio Adan left bewildered.
Real Betis were 2-0 up at the time and Valencia down to 10 men after Enzo Perez was sent off, but Munir's introduction sparked a comeback, with Rodrigo scoring after a Dani Parejo pass in the 75th minute.
Munir forced Adan to save his header from a corner three minutes later and Ezequiel Garay battered in the rebound.
Unfortunately for Munir, Castro had the last laugh, tucking home in stoppage time to win his team the game.