Jose Mourinho's side go into their Champions League Group H clash having failed to win any of their last three matches, and with only four wins from nine games in all competitions this season.
That form has left Mourinho under intense pressure but when asked about influential voices in the dressing room, Matic told reporters that it is on the field where United need inspirational figures.
"I don't think you can resolve something with talking," he said. "The leaders need to be on the pitch.
"The leaders are the players who aren't scared to play. We will see tomorrow and in the next games who the leader is.
"If you talk in the dressing room, that doesn't mean anything. The most important is on the pitch."
United's 3-1 defeat at West Ham on Saturday was their third in the Premier League this season and they are 10th in the table after seven games.
A poor result against Valencia would lead to even more scrutiny on Mourinho's position but Matic said the added pressure was not a concern to his team-mates.
"Pressure is always there," he said. "All my life I have played with pressure and also my team-mates all play with pressure.
"We're not scared to play. We play football but everyone is trying to win, especially against Manchester United.
"We know that and we have to be ready for that. We weren't ready in the last few games. As soon as possible we have to be ready."
Mourinho, whose rift with Paul Pogba has been well publicised in the last week, admitted on Monday that some of the players in his dressing room care more than others about United's recent run of results in what was a thinly veiled dig at his squad.
The Reds' boss also confirmed that they will be without Jesse Lingard, Ander Herrera and Ashley Young for the Valencia clash at Old Trafford after the trio picked up minor knocks.