"If surveys are now talking about 22-23 seats for Netanyahu, that could easily end up being only 18," the unnamed source was quoted as saying by the publication.
Last week's opinion polls across radio, newspaper and online media forecasted Likud to gain 22 to 23 seats, with its allied nationalist Yisrael Beiteinu party slated to get five to six.
The reason behind fears of lower-than-expected results, as cited by the Haaretz, is Likud's nearly two-decade history of winning 10-30 percent fewer votes than depicted in the polls.
The Knesset voted to dissolve itself following an unresolved dispute within Netanyahu's ruling coalition in December 2014.
The early parliamentary election is scheduled to take place two years before its intended date on March 17.
Likud's main opponent is the center-left Zionist Union, which is a single-ballot merger between the Labor party (15 seats) and the dismissed justice minister Tzipi Livni's Hatnuah (six seats) party.
