On Thursday, Mattis had announced his resignation after Trump’s sudden announcement of withdrawing troops from Syria and Afghanistan. He had rebuked Trump’s foreign policy in his resignation letter.
Mattis has criticised Trump for failing to value America’s closest allies, who fought alongside the US in both conflicts.
Trump announced that Deputy Defense Secretary Patrick Shanahan would take over for Mattis on an acting basis on January 1. In a tweet, Trump called the former Boeing Co executive "very talented and said about Shanahan "He will be great!"
The official said Trump was expected to pick a nominee for defense secretary over the next couple of weeks.
Mattis’ resignation has come as a reflex to Trump’s decision of pulling out troops from Syria and Afghanistan. The US President's decision attracted widespread criticism from the lawmakers, think-tank community and experts.