The defender is wary of the threat posed by the Red Devils’ speedy attackers after last month’s shock derby defeat at the Etihad Stadium
The 18-year-old defender has played in every round of the competition so far and, after starting City’s last two Premier League matches, will hope to be involved at Old Trafford.
City were caught out by the speed of United’s counter-attacks in a 2-1 defeat in December when Marcus Rashford and Daniel James caused them huge problems.
But Garcia says they will continue to try to dominate possession and play high up the pitch even though they know where United’s threat will come from.
“Obviously they are very dangerous on the counter-attack because they have really, really fast players,” he said.
“We need to try to avoid that because at the end it could be the key for the game.
“I think we need to play like we always play – trying to attack and playing very high.”
“Recently we’ve been playing teams that really like the counter, especially United, so we need to be careful with the balls that we lose in every single area and try to avoid that and be secure at the back.”
Ole Gunnar Solskjaer rested many of his senior players in Saturday’s FA Cup stalemate with Wolves, including Rashford, but Garcia warned about the threat from rising star Mason Greenwood, having come up against the 18-year-old in youth football.
“He’s a player that doesn’t even need a chance to score,” the former Barcelona youth player added. “He’s good with his left and right foot so he’s a player to look out for.”
City boss Pep Guardiola has recently switched to a back three, with Garcia on the left-hand side of midfielders Fernandinho and Rodri.
John Stones is also available for Tuesday night’s first leg, while Aymeric Laporte is stepping his recovery from the serious knee injury that has disrupted much of his season.
But Guardiola says he could continue with three central defenders even when he has his full squad fit as it gives his full-backs the opportunity to get forward more frequently.
“In some games [it’s been] good, in some not good, it’s nice to have alternatives, especially for the quality of the players we have,” the City boss said.
“Especially in the back to adapt better, to control the build up, to control better the counter-attacks, to have more passes, that’s the idea.
“It’s a way we can play with the full-backs that we have, especially Mendy, Kyle [Walker], Joao [Cancelo] and Angelino.”