Timo Glock says he feels "sorry" that quadruple world champion Sebastian Vettel's achievements are being diminished.

Eddie Jordan, a former F1 team owner turned outspoken television pundit, told the F1 Nation podcast that he would "probably not" sign up the Ferrari refugee for 2021.

"Can he rekindle a kind of sparkle and a fire and the enthusiasm and the charisma that he had? It's going to be difficult at his age," Jordan said.

Glock, a former F1 driver, admits that Vettel's last few seasons with Ferrari have damaged his reputation.

But he says the Maranello team lacked the stability to allow the German to emulate the feats of his mentor Michael Schumacher.

"With Seb, things were a little lacking. In his time, there were constant changes at the management level," Glock told the Konigspartie podcast.

"He didn't get the opportunity to build a team around himself to make Ferrari as successful as Michael was able to."

Glock said Vettel not having a manager was another obstacle.

"It's never good for a driver to try to turn things upside down with a lot of pressure. It's always easier if you can send someone else to do the kick-off," he said.

"It's an area in which he wasn't well positioned and thought he could sort it all out on his own because he was used to that at Red Bull. It was easier for him there," Glock added.

Nonetheless, he thinks those in Formula 1 are too quick to forget Vettel's achievements.

"It's a pity and I am sorry that there are those who forget that he is a four-time world champion, and that in the first few years he definitely put Ferrari back in a position to fight for the title," said Glock.

"Now, the mistakes always stand out before you think about the positives. That's difficult as it puts you under even more pressure and creates even more mistakes."