Deportivo defender Fabricio Coloccini and Arsenal centre-back Philippe Senderos are reported targets.
Midfielder Geremi recently voiced his concerns about Newcastle's thin squad and Keegan acknowledges a need to reinforce.
"I would still like to strengthen and I'm confident we'll bring in at least two, possibly three, more faces," he told the club's official website.
"And if those players are of the quality of the likes of Gutierrez and Guthrie then we'll take our chances."