It is currently unrealistic to achieve synergies of 1 billion euros ($1.4 billion) or even 500 million euros in a merger between MAN and Scania, Leif Oestling told the Sunday edition of Welt am Sonntag in comments released a day earlier.
The newspaper said he was interviewed in November before MAN reorganized its top management.
MAN named a new chief executive and other board members on December 11 barely three weeks after three of its top executives, including CEO Hakan Samuelsson, left the company.
The executives' departure has fueled speculation that Volkswagen (VOWG.DE) would make a bid for the German truckmaker.
Volkswagen Chairman Ferdinand Piech, who is also chairman of MAN, has hinted this year that MAN is on his radar screen as he tries to revive plans for a three-way truck alliance with Swedish unit Scania (SCVb.ST).
VW holds around 70 percent voting rights in Scania and 29.9 percent in MAN, which in turn owns a 17 percent stake in its Swedish rival. Oestling told Welt am Sonntag any merger with MAN would make little sense from an industrial point of view and that it could damage the MAN and Scania brands.
Commenting on the overall truck sector, he said: I expect that we have already reached the bottom but that new record levels in the truck market would only be reached between 2015 and 2020.