Chelsea went top of the table when they beat Leeds on December 5.
Since then, they have lost five of eight games – and Lampard is now feeling the heat.
Hiddink told the Mirror: “When it comes to signing players, there is a number of people who are responsible.
“A coach, of course, is part of the process – but it is also about the policy of the club to decide what kind of players you want to attract.
“Many factors have to be considered before making a judgement.
“But the ultimate responsibility for results always lies with the coach – and Frank will understand that."