The revelation came during an interview on MSNBC during which the Democratic senator was asked for his opinion on the flurry of executive actions passed by the president since Wednesday.
Senator Coons, who holds Mr Biden’s old seat and has known the president for 30 years, said the president had been thoroughly preparing since he won the election in November.
“Mr Biden has been digging into details, pulling together a seasoned and capable and diverse group of nominees to serve as his cabinet and preparing for this moment,” he said.
However, he noted that “unfortunately although not surprisingly” transition teams had discovered that “outgoing president Trump and his team caused more harm than expected.”
“Not only was the cupboard bare in terms of leadership but it was shockingly worse than they expected,” he said.
Mr Biden signed a flurry of executive orders on Wednesday following his inauguration before the celebrations of his appointment had even concluded.
The new president passed at least 17 executive orders and a range of new domestic and international policies on issues such as climate change, coronavirus, and immigration.
Mr Trump and his family refused to attend the inauguration ceremony following a bitter transition period during which he continued to baselessly claim that Mr Biden had been fraudulently elected.
The president refused to concede the election until January and delayed the beginning of the transition period until weeks following the election, a move that many feared would impact the new administration.
Senator Coons said in particular that there “really isn’t a detailed plan for how to roll out vaccines to hundreds of millions of Americans.”
The previous administration fell short of delivering 20 million vaccine doses by the end of 2020 to citizens across the country, with around 16.5 million vaccine doses administered as of 20 January, according to the Centres for Disease Control and prevention.
Mr Biden has vowed to move “heaven and earth” to administer 100 million vaccines in 100 days since his inauguration as president on 20 January.
“We here in the Senate need to also meet this moment by promptly confirming his cabinet and his senior leadership team that requires senate confirmation,” Sen Coons said.
“And we need to come together quickly to move on the American rescue package that president Biden proposed now a week ago.”
Mr Biden unveiled a $1.9 trillion Covid relief plan that includes $1,400 cheques for most Americans as he prepares to boost the government’s pandemic response earlier in January.