Bota assemblyperson Nasaruddin Hashim left Umno yesterday to join Pakatan in Perak, prompting speculation that others might also follow suit.
"These are all rumours... I don't think so," said Najib.
According to a Bernama report, Najib described Nasaruddin's action as unexpected.
The incoming Umno president said that he hoped Nasaruddin could think carefully his decision to join PKR, a Pakatan Rakyat component member.
He added that he was made to understand that Nasaruddin's action was due to several factors.
He, however, did not elaborate.
Najib was talking to reporters after attending the MCA's Chinese New Year open house in Kuala Lumpur here today.
Later, at the Gerakan open house, Najib said Nasaruddin's decision took many by surprise.
"We heard about this two days ago but it is still a shock," he said.
Najib said Umno and BN elected representatives should understand that the party had done a lot of good deeds to the people and the country.
"It is not right to abandon the party because of personal problems or dissatisfactions at a time when the party needs our support," he said.
With Nasaruddin's defection, the Perak Pakatan Rakyat government now has 32 seats compared to BN's 27.In the March 8 general election,
Nasaruddin, who polled 6,556 votes, defeated PKR candidate Capt (Rtd) Usaili Alias with a 2,068-vote majority. - Malaysiakini