Eduardo, who spent three years with the Gunners up until 2010, feels that the duo's departure has lessened the club's chances of finishing within the top four of the Premier League this season.
"It is a big mistake to sell them. They were the most important players last season," he said. "Every player comes to Arsenal as a simple player and Arsenal make them into a big player.
"But they can't keep everyone together to fight for titles. Every year it's similar. They lose players, buy players, make them class players and the next year they sell again. They are always in the same place."
"Van Persie and Song were the key players last season. They held the team. When you lose two players like that, your chance of finishing in the top four is less."
Arsenal drew their opening match of the season 0-0 with Sunderland on Saturday afternoon.