The Spanish side claimed a 3-0 victory over BATE two weeks ago to join the Belarusian side and Bayern Munich on six points in Group F but Pellegrino is expecting a different challenge in the return fixture.
"BATE have played very well away from home, they are comfortable sitting back and waiting for their opponent to make a mistake, that is one of their principal weapons. We will need to be fully switched on," he said.
"It is a very important game, given that the group is very open. If we win it will be a big step forward ahead of the two next games. If not the situation will become pretty difficult. A draw would leave us level with two games to go."
BATE coach Viktor Goncharenko does not believe his players will be scarred by the 3-0 reverse they suffered at home to the two-time Champions League runners-up.
"I'm not worried about the game because all the players who have come are highly motivated to improve," he said. "Each coach is to find the strength of the opponent, remove it and take advantage of weaknesses."
Goncharenko has seen his side win their other two Champions League matches so far but does not want his players to start thinking of the knockout stages just yet.
"We must eliminate the mistakes players made against Valencia in Minsk," he said. "The group is still open and so we cannot think beyond this game."
Pablo Piatti is available again after two months on the sidelines with a broken arm, with the Argentinian forward included in Mauricio Pellegrino's 20-man squad.
Pellegrino is also hoping to call on Adil Rami after the central defender was forced off in the weekend win over Atletico Madrid due to injury.
BATE will be without midfield duo Aleksandr Pavlov and Filipp Rudi after they remained behind in Belarus for treatment on minor injuries.
Coach Viktor Goncharenko has taken a 20-man squad to Valencia, however, and faces a tough decision over who to leave out.