Mitchell struggled in the charts after the first series ended, only reaching number 45 with 'Run to You', a cover she performed on the BBC One show's final.
The Black Eyed Peas star said that the handling of Mitchell had been done in a way which would have been appropriate "five years ago".
"I don't think people were considering today's behaviour and how we digest content. It was television five years ago. I hope we've learned from that because I'm part of this show. I hope the label that the artists have to go with have learned from it," said will.
"So with this year's winner, we already have songs ready to go. The label need to have an understanding of what the market is thirsting for and yearning for."
Explaining why he thought Mitchell's career had got off to a slow start, he said: "We had a hole in the system. There were all these people singing on television and then one of them wins. Directly after they win - I don't know what happened. I don't know if the label engaged and surrounded it.
"I don't think the rushing happened and if it did, it was too slow. Especially in this day and age when everything changes every 20 seconds on Twitter. You go from something being hot to 'Oh, it's not hot anymore'."
Watch our exclusive chat with Voice coaches will.i.am and Sir Tom Jones:
Speaking about lessons he had learned from series one, will said: "You have always got to be hunting and searching for what you're going to do next week, next month, next year. We should be thinking about the winner of The Voice [series 2] right now. I have a song written right now."
Tom Jones, who mentored Mitchell on the show, admitted that he would have liked to have worked more closely with his acts after the show ended, but said that it wasn't possible because of record label contracts.
"The problem we have as coaches is that we have no say in what they do. They are locked up," said Jones.
"We help them to where they want to get in the show. I would have liked to have carried on working with a few of them. But all you can do is show them where to go."
He added: "The thing is when you win, you are signed then to Universal Records. It's just a start. It's just a start for her. You've got to do an album and hopefully have enough say in it yourself."
Jessie J added: "[Winning the show] doesn't guarantee success. We have to prepare them. That's all we can do."
Universal Music had no comment on will.i.am's remarks about Leanne.
The Voice returns on Saturday, March 30 at 7pm on BBC One.
Leanne Mitchell releases her debut album through Universal Music on May 27.
Listen to Leanne Mitchell's new track 'If I Knew Then':
The Voice series two new pictures - gallery: