Guthrie, who had been Lauer's co-anchor since 2012, took the news particularly hard, a source tells ET.
Before the bombshell allegations of sexual misconduct that led to Matt Lauer's firing from the Today show by NBC, he and former co-host Savannah Guthrie had grown close professionally in their five years of working together.
The 45-year-old morning show host, who made the announcement of Lauer's termination on-air last Wednesday, was "affected the most" by the news, a source told ET last week.
"She’s not herself. She’s visibly shaken," the source added, explaining that around the studio on Thursday morning, Guthrie appeared "sad, in deep thought and preoccupied" off-camera. "She is doing her best to stay upbeat and jolly on-air."
Guthrie and Lauer had shared the desk since July 9, 2012, when she took over for Ann Curry. Since then, the two, along with Al Roker, also hosted both the Macy's Thanksgiving Day Parade as well as the Rockefeller Center Tree Lighting ceremony.
In July, when Guthrie celebrated her five-year "Savannahversary" with Today, Lauer was among the co-hosts to offer her praise, while celebrating in studio with her favorite milkshakes and cake.
“You make it look way too easy,” Lauer shared.
"I think Savannah is one of the most well-rounded people-slash-personalities on the program," Lauer gushed, in a retrospective feature on Today about Guthrie's five years with the team.
Seeing how long they were on the show together, it's not surprising that they shared many memorable moments including two major milestones that have happened during Guthrie's tenure on Today -- the birth of her two children.
"Vale Guthrie Feldman is here! Savannah and Mike have a beautiful (and smart!) little girl!" Lauer tweeted in 2014, when Guthrie's daughter, Vale, was born. "So much joy and excitement. We love her already!"
This past March, Guthrie brought Vale, as well as her then-3-month-old son, Charley, on the show to meet Lauer and the rest of the Today show family.
Obviously, the tone on set has certainly changed in the wake of Lauer's firing after more than 20 years with the show. Lauer was fired by NBC after the network received a "detailed complaint" about his alleged behavior from a colleague. Since then, multiple women have come forward accusing the former anchor of inappropriate sexual conduct.
In response to the allegations, Lauer denied some of the claims, while expressing remorse for past behavior that he now regrets.
"As I am writing this I realize the depth of the damage and disappointment I have left behind at home and at NBC. Some of what is being said about me is untrue or mischaracterized, but there is enough truth in these stories to make me feel embarrassed and ashamed," he said in a statement. "I regret that my shame is now shared by the people I cherish dearly. Repairing the damage will take a lot of time and soul searching and I’m committed to beginning that effort. It is now my full-time job."
"The last two days have forced me to take a very hard look at my own troubling flaws. It’s been humbling. I am blessed to be surrounded by the people I love. I thank them for their patience and grace," he concluded.