Daniel Ricciardo's career is at risk of losing momentum.
And after four races in 2019, Webber said it is now clear that Renault has a lot of work to do to be competitive.
"Daniel hasn't forgotten how to drive, but the form guide changes very quickly in this sport," Webber told Australia's Nine Network.
"Every few months, if the performances aren't there, other managers and agents and young drivers are trying to muscle in. The sport moves on so fast," he added.
Webber, who retired from F1 at the end of 2013, said Renault is under "tremendous pressure" because the performance so far this year "doesn't back up the chat".
"Renault were saying in the off-season they can challenge the top three, but they're a long way from that," he said.
However, Renault team boss Cyril Abiteboul is putting on a brave face, even though amid a bad weekend in Baku, the team dropped to seventh in the constructors' standings.
"I think it's fair to say that it's not exactly the start of the season we were hoping to have, but I think it's also important to take a bit of distance from the emotions and the constant drama of Formula 1," said the Frenchman.
But Webber thinks the situation for Renault is indeed dramatic.
"They've got some reliability issues as well, McLaren are belting them with the same engine, which is not good," he said.
Webber said he doesn't relate to Ricciardo's decision to move from a truly top team like Red Bull to the deep midfield.
"I was in average teams for a long time and I never, ever wanted to feel that again, even though I had offers to continue when I left Red Bull," he said.
"I didn't want to know what that felt like again."