The Liga leaders failed to win a home league game for the first time in a year, with Maxi Gomez's equaliser for the visitors coming after Umtiti pulled up injured.
The French defender is set to miss El Clasico against rivals Real Madrid on December 23, while Andres Iniesta was also replaced with an unspecified problem shortly after half-time on a bad day for Barca, which was later compounded by Atletico Madrid's last-gasp home win over Real Sociedad.
"We had the chance to go 3-1 up and then that situation [Umtiti's injury] cost us two points, as normally you have an extra defender," Valverde said.
"Samuel had to go off. We tried, we hit the woodwork through [Gerard] Pique, the chances for Paco Alcacer, Paulinho and Leo [Messi].
"That move saw the culmination of several pieces of misfortune with the equaliser and the injury.
"That was the way the game was and Celta are a good side and had the ability to equalise.
"To lose Samuel is a big blow for us as he has been playing well.
"I don't know how bad the [Iniesta] problem is but it is no way comparable to that of Umtiti."
Siempre juntos, positivos y mirando hacia adelante!!! Força Barça— Luis Suarez (@LuisSuarez9) December 2, 2017
Always together, positive and looking forward!!! Força Barça pic.twitter.com/0OdDjHZqaK
"It was a close game where they started well and we found it difficult to get involved," said Valverde.
"After their goal we became more incisive and got the equaliser.
"After that we started to dominate and in the second half we got the second goal but then came that unfortunate move which cost us."