The Barca captain fractured the radial bone in his right arm in the first half of their 4-2 defeat of Sevilla last night after falling awkwardly.
The Argentine had provided an assist for Philippe Coutinho’s opening goal and scored the second himself before then to put his side in command.
Barca went on to claim a much-needed win to end a four-game run without a win in La Liga.
But their triumph came at a cost with Messi now likely sidelined until after November’s international break.
And Valverde acknowledged the psychological boost that will offer their opponents given the threat the Argentine ace poses whenever he is on the pitch.
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The Barca boss admitted: ”Of course he’s a major loss. We know what he does for us and what our opponents think just by seeing him on the pitch.
“We must prepare ourselves. We have the players to cover for him, but we have some very tough games coming up.
“In the past we have had games without him and have always tried to maintain our style.”
On the result, Valverde reflected: “It’s very important for us to have regained top spot in the league.
“This time it was us who got an early goal. The injury meant we lost some of our concentration.
“We played with just ten men for quite a while because we were hoping we could get Messi back out there.”
Luis Suarez ended a seven-game wait for a goal with an important performance to help get Barca over the line in Messi’s absence.
And Valverde revealed how pleased he was with the Uruguayan front-man but insisted his performances have not wavered despite his lengthy goal drought.
He continued: “He’s the same as ever, fighting hard, creating chances and a constant headache for opponents.
“He got involved tonight, scored a goal, and we’re happy with him.”
Barca now prepare to face Inter Milan in Europe in mid-week before a huge showdown with Real Madrid in the Clasico next Sunday.
Los Blancos have lost four of their last five matches with pressure increasing on the under-fire Julen Lopetegui, although Messi’s absence will too impact on the tie.
Valverded added: “This is an important week for us, we play for the lead in our [Champions League] group with Inter and then it is the Clasico.
“We have high hopes for this week. It is difficult as there are two important games.
“But I look ahead to these games just like the players. Noise from the outside about them is normal and nothing can be done about it.
“We are just focused on winning against Inter and then trying to beat Madrid and we will see if we can do that.”