Analysts said the Heijmans bid put a fairly low value on HBG, leaving it open to a rescue bid from another company.
One said: "It makes HBG vulnerable because it puts the spotlight on it. If people think the bid is undervalued you may get a white knight coming in with a slightly better offer."
But he added that HBG's exposure to the stalling German market may put bidders off.
HBG has a £919m-turnover construction firm in the UK.