The 1986 World Cup winner has doubts that the Catalans can eliminate Milan, and believes United's defensive approach against Madrid cost them
The Blaugrana were beaten 2-0 in the first leg of the tie at San Siro in February and they will be looking to turn things around at Camp Nou on Tuesday, and the Argentine is curious as to how they will respond.
"Barcelona's confidence has fallen into the abyss in the space of just a week," Valdano was quoted as saying by Marca.
"We don't know how Barcelona are going to respond to adversity; they've never faced it before. Maybe we have underestimated the importance of the coach at Barcelona."
The Argentine then looked back at Real Madrid's win over Manchester United earlier this week, and voiced his opinion that the Red Devils were too focused on defending their lead.
"When Manchester United scored the goal they just sat back, but there was still a lot of time left."
Madrid booked their ticket for the quarter-finals by beating United 2-1 at Old Trafford after recording a 1-1 draw in the first leg.