Athletic are at Old Trafford for the second-leg of their Europa League semifinal, needing to overturn a 3-0 deficit.
Valverde said in his media preview on Wednesday: "We're thinking about those who have come, focusing on those who are going to be here.
"United will also have absentees, the important thing is those who are on the pitch. We're a team and we have to deal with the absentees we have.
"It's complicated, the result is adverse and they're a great opponent. This is football, anything can happen and we always hope for the best."
XI v XI will be a different game
Looking back on the first-leg, Valverde says Dani Vivian's red card made the difference.
He said, "With 11 against 11 we were closer to winning, but Man Utd don't forgive and we made mistakes. They're a team with a lot of ability, so we hope to make fewer mistakes.
We're very satisfied with how we've approached this competition.
"We know perfectly well that in this type of tie everything can be decided in one minute or by one wrong move. We'll be ready to take advantage of any opportunity."