Simeone's men were second best for much of a cagey quarter-final first leg with their city rivals at Estadio Vicente Calderon on Tuesday.
Yet they were able to stand firm for a goalless draw thanks in part to a fine goalkeeping performance from Jan Oblak, setting the stage for a tense second leg at the Santiago Bernabeu next Wednesday.
"Diego Simeone's Atletico Madrid is a different beast to the team I played for," former Atletico striker Forlan wrote in his column for The National.
"The first thing he did when he arrived, soon after I'd left the club in 2011, was fix the defence. He said he wanted to see an "aggressive, strong, combative and determined team", and his impact was immediate, with six clean sheets in his first six games.
"They had not managed one in the nine games before he took over.
"When I played for Atletico, the team had an attacking mentality. Myself and Sergio Aguero, with support from Jose Reyes and Simao, were all expected to get goals, and we did.
"I got 35 in 2009 and 28 a year later. Aguero scored over 20 both years, too. In one game we had scored four past Barcelona, but conceded three.
"Simeone changed that. Defence is vital to his teams as we saw against Real Madrid in the Champions League on Tuesday.
"The teams have met seven times this season and Madrid [Real] haven't won once. They were so dominant in the first half of the Champions League game on Tuesday, but the ball would not go in.
"Atletico defend with such confidence, even against a team like Madrid. It wasn't a surprise.
"It will be hard for Atletico on the bigger Bernabeu pitch, and I’d expect them to defend again. But I wouldn't be surprised if they counter-attacked, scored and went through. But then let's not get carried away."
