McLaren is yet to sign an engine deal for the new turbo V6 era beginning in 2014, according to Germany's Auto Motor und Sport.
"Mercedes is pushing for a decision in the next days," said correspondent Michael Schmidt.
The successful, title-winning McLaren-Mercedes partnership dates back almost two decades, but with Mercedes now with its own team, McLaren is no longer the 'works' partner.
According to Schmidt, the solution could be Honda.
"The question is whether the Japanese are coming in 2014 or not until 2015," he said.
"It is a poorly kept secret that McLaren is talking with Honda."
In fact, Schmidt relayed paddock rumours that Honda already has engineers working at McLaren's Woking headquarters, with a final board decision about whether the manufacturer will return to F1 after a half-decade absence "pending".