McLaren must bounce back in Germany: Prost
McLaren stormed to the top of the constructors' standings after strong results in the first three races of the year, but have scored only one podium finish in the six races since then.
"If they don't come back immediately, from Hockenheim, the next race, I think it's going to be difficult," Prost said.
"In front, Red Bull and Ferrari are very motivated and it's going to be tough."
One of the reasons for McLaren's decline is that they have failed to cope with the new tire compounds introduced this season, he said.
"It looks like they do not understand what is happening with the new tires, which is not good news."
Ferrari's Fernando Alonso leads the drivers' championship by 13 points from Red Bull's Mark Webber, who won the last round in Britain, while Webber's teammate Sebastian Vettel is third with Lewis Hamilton fourth for McLaren.
In the constructors' standings, Red Bull have a commanding 64-point lead over Ferrari, who enjoy a narrow advantage over third-placed Lotus and fourth-placed McLaren.