Valverde said the 21-year-old was left behind after "sports assessment".
Messi has returned as the main thrust of Barci. Their captain was involved in the travel team in Inter's face on Wednesday but did not think it was sufficient even for the bench.
Asked whether he could play 90 minutes against Betis, Valverde said, "We'll see, but he has lost fear of falling or fighting against the opponent.
"You have to get it out of your head, you can not think that you are going to make a bad move. I saw him training well and with certainty."
Messy absence should be a golden opportunity for Dembel, but winger's form has been mixed in recent weeks, while Luis Suarez, Philippe Coutinho, Rafinha and Malcom impressed.
"Everyone can interpret it as they want, but this is a team," Valverde said. "I'm looking for the best team and that's all."
Barcelona has plunged about 105m euros to sign Dembel from Borussia Dortmund last year, but injuries and inconsistencies have hurt their career in Spain so far.
"Let's hope he can use all his talent for the team to benefit and the club that has played a big part in it," added Valverde.
Umtei was a key player to Barca, but the defender's starting lineup this weekend was not guaranteed, as Clement Lenglet did well with Gerard Pique during a strong team squad.
"The big news is that Lenglet showed his level," Valverde said. "Far from being a problem, it is perfect to have more opportunities, tomorrow we'll see who plays."
Barcelona sees three points at the top of the league and is hoping to win another 14th place at Betis before the start of the international break.
Valverde also praised his full back Jordi Alba, who for the first time reminded him of the Spanish team under new coach Luis Enrique.
"We've seen how Jordi improves every day," Valverde said. "Every coach is looking for something and his start of the season is spectacular. Call for the team is deserved."
Coutinho will not play against Betis. The giver returns from the cartilage ridge.