The South Americans went down 3-2 to an impressive Dutch outfit at the Cape Town Stadium on Tuesday evening.
Forlan was substituted with five minutes to play and he admitted that he was not at this best during the clash after going into the semi-final tie with a tendon strain.
"I have had a problem with my tendon, this has been going on for a couple of days already and I wasn't really in the game," he said.
"I thought warming up before the game I was going to be at my best, but it wasn't like that for me."
The former Manchester United star says he was gutted not to be at full fitness and laid the blame on a gruelling season which has seen him play 67 games.
"This was the most important match of my career and I wasn't 100 percent fit, I feel bad that I wasn't able to give my best," he added.
"I'm tired, my body couldn't give any more. I have played 67 matches this season and it has just been too much."