The Dane admitted at Hockenheim that speculation he and teammate Jolyon Palmer will lose their places for 2017 was "annoying".
Denmark's BT newspaper says it knows that Magnussen's backer, billionaire Anders Holch Povlsen whose 'Jack and Jones' logo is on the yellow car now, remains committed.
Magnussen's sponsor agent, Thomas Badura, would not comment on that, but he said there is "nothing to suggest" that the 23-year-old's current commercial arrangements are set to change.
"Both current and potential sponsors are aware that 2016 is the first step," he said.
Still, Renault team boss Frederic Vasseur insists that the 2017 lineup is not yet decided.
"I have the advantage that 70% of the drivers on the grid have driven for my previous team (ART) and I talk to them every single day," he said.
"But I also speak to Jo and Kev every single day as well. The difference is that they should focus on their jobs, as we are far from the time when we take our decision.
"There definitely will not be a decision before September. Definitely."
He also admitted that sponsorship may also play a role in the final decision.
"We are a company," said Vasseur. "Right now it's all about performance, but at some point we also speak about sponsors. It may prove important."
The 2016 championship resumes after the mid-season break on August 28 in Belgium.