"I got to know his wife, Judy, and had met her and shared her pain. I met with him when he was in prison in Havana and I really feared for his health if he continued to be in prison," he said.
The White House also informed Senate Majority Leader Harry Reid, Sen. Barbara Mikulski (D-Md.), Sen. Ben Cardin (D-Md.) and Sen. Patrick Leahy (D-Vt.) of the release Tuesday night, but it was not immediately clear whether top Republican leaders had also been informed.
Other lawmakers closely tracking U.S.-Cuba policy said they were given just a 30-minute advance warning that a "major announcement" regarding Cuba policy was imminent.
After keeping close tabs on the situation over the past several years, Durbin said he learned only recently that the Obama administration and the Vatican have been working together for more than a year to secure Gross's release from Cuba.
“I’ve spoken to this president many times over the years and I know that it was his goal to change our relationship with Cuba," he said in an interview. "I also knew that the administration was quietly behind the scenes doing a lot to free Alan Gross."
Durbin said he traveled to Havana in 2012 with other Democratic senators to meet with Gross at a Cuban prison hospital and has been concerned with his plight ever since.
"He was in a prison hospital setting. But it was – I just couldn’t understand how a person who simply brought computer equipment in for Cuban Jews at a synagogue would be held in those conditions for five years. I really worried about him," said Durbin.
During their conversation, Durbin said that Gross had told him he believed "he was a pawn in this prisoner exchange."
Durbin, the second-ranking Democrat in the Senate, acknowledged that Wednesday's dramatic announcement is likely to spark a fierce debate on Capitol Hill about the future of U.S.-Cuba relations. But Durbin said he's ready for the fight.
"I think most will acknowledge that our foreign policy for over half a century has not been successful. We had hoped by excluding Cuba and pressuring Cuba that the regime would change and it never happened. I think this opening the door to free travel and trade and exchange of realities is going to have a more positive impact in changing Cuba than 50 years of foreign policy," he said.
Durbin noted that a few of his colleagues, including Sens. Robert Menendez (D-N.J.), Marco Rubio (R-Fla.) and Ted Cruz (R-Tex.) have personal family ties to Cuba "that are a major part of their thinking about Cuba today. I respect that very much, but I just believe at the end of the day that this new approach is the best way to change Cuba for the better."