The Chelsea striker has a reputation in England as a dirty player who can be aggravated by some close-checking defence.
This season, the Brazilian-born Spaniard has honed his aggression and focused on what he does best – score goals – which has helped the Blues to a 12-game winning streak and top spot on the Premier League table.
“One player who has been doing exceptionally well is Costa, the Premier League’s top scorer,” Forlan wrote.
“We both joined Atletico Madrid at the same time in 2007 and I played alongside him a couple of times towards the end of my time at Atletico, but he was still a teenager when he arrived and kept being sent out on loan.
“He went to Braga, Celta, Albacate, Valladolid and Rayo Vallecano, which is normal for an emerging player who is not quite ready to play every game for a top team. My departure in 2011 helped create a space for him.
“I liked Diego. He was a funny guy, always joking about with the other Brazilian and Portuguese players there, always smiling and completely different to the man people see on the pitch, the fighter with the strong personality
“It is like he transforms when he crosses the line.”