Norbert Haug in Turkey said it would be pointless to try to keep a driver if he wants to go elsewhere in Formula One.
"You can't stop the migration of birds," the Mercedes chief is quoted as saying by the German press.
Haug, however, insisted that "at the moment there is no evidence that one of our drivers is leaving us".
But he said the movement of drivers is just part of Formula One.
"The fact that in principle we can look around for who might come to us some day, and that the drivers look as well, is completely legitimate and part of the business," added Haug.
The theoretical departure of Rosberg would create an opening for Paul di Resta, the impressive Mercedes protege who is shining on his 2011 debut with Force India.
At the moment I am concentrating only on the challenge of moving forwards
"The link to Mercedes is not just idle speculation," di Resta's Scottish countryman David Coulthard wrote in his Telegraph column from Turkey.
"Paul has made a great start to his F1 career and looks like he belongs," he added.
German Rosberg, however, played down the prospect of a vacancy any time soon.
"I don't want to say anything about the rumours. At the moment I am concentrating only on the challenge of moving forwards with this team," he is quoted by the Cologne Express.