Pique, who spent four years at United as a youngster, returns to Old Trafford this week for the first time since moving back to Barca in 2008.
Barca start the tie as favourites, but United come into the clash following an impressive comeback against Paris Saint-Germain in the last round.
Pique says he watched both games against PSG and urged his teammates to take the chances which come their way, to avoid suffering the same fate as the French champions.
“I think the PSG tie is a great example.
“I think [at Old Trafford], PSG played better. In the second half, they scored and could have scored more.
“But you can’t leave United alive. If you can beat them [by] scoring a lot of goals, you have to do it. They have a great history and they showed that in Paris, even with a lot of young lads from the academy. It’s a great example for us. We have to be switched on from the first minute until the last,” the Spain defender said in a news conference on Tuesday.
