On Saturday, r abruptly replaced CEO after nearly a decade at the helm, based on his worsening health.

The company has been notably reticent with details about Marchionne's condition, but his health appeared to worsen after undergoing shoulder surgery approximately three week ago. The Italian-born 66-year old, who led the company out of its government bailout into an impressive era of prosperity, will also step down from his dual role as CEO of , the race car manufacturer spun off from Fiat more than 2 years ago.

The move to replace Marchionne came just days before Fiat Chrysler is scheduled to report second quarter earnings. In an emotional letter, company president John Elkann hailed Marchionne's tenure, sought to reassure employees about the future, and rally them around newly appointed CEO Mike Manley. The jeep executive will usher Fiat Chrysler into its new era.

Below is the full text of the communication Elkann sent to staffers, obtained by CNBC on Sunday:

Dear Colleagues,

This is without a doubt the most difficult letter I’ve ever had to write.

It is with great sadness that I have to tell you that our CEO Sergio Marchionne, who recently underwent surgery, unfortunately experienced complications that have worsened in recent hours and will prevent his return to FCA.

For the last 14 years, first at Fiat, then at Chrysler, and finally at FCA, Sergio has been the best CEO that anyone could ask for and, to me personally, a true mentor, partner and close friend.

We met at one of the darkest moments for our company and it was his intellect, perseverance and leadership that saved Fiat. He also achieved a remarkable turnaround at Chrysler and, through his courage in forging the cultural integration of the two companies, he established the foundations for a more secure and brighter future for us to take forward. For what Sergio has been able to accomplish, turning the impossible into the possible, we will be forever grateful.

But as he himself has said more than once, “The true value of a leader is not measured by what he has gained during his career but rather by what he has given. It is not what you accomplish today, but the legacy you will leave behind.”

Since the first evening we spent together, when we talked about the possibility of him taking the helm at Fiat, what really stood out to me, beyond his exceptional and extraordinary professional ability, were his human qualities. I saw these qualities in his eyes, in his demeanor, in the way he dealt with those around him.

He taught us to have the courage to challenge the status quo, to break with convention and go beyond the tried and tested. He has always pushed everybody to learn, to grow and to excel – often beyond their own limits – starting always with himself. The legacy he leaves with us speaks to what really mattered to him: the pursuit of excellence, the idea that there is always a better way.

His lessons, his impulse never to accept the status quo, never to be satisfied with good enough have become a core part of our culture within FCA: continually setting high standards and never settling for mediocrity.

Sergio’s definition of a leader is truer today than ever. What really matters is the kind of culture he leaves behind. The best way to judge a leader is by what the organization does after him.

This is yet another example of the true, rare kind of leader Sergio was.

We started years ago working on a succession plan that would ensure continuity and preserve the unique culture embedded in FCA.

With that plan already established, we are now moving ahead with the process and today the Board of Directors has named Mike Manley as the new CEO of FCA.

Mike has been one of the major contributors to FCA’s success and he has a remarkable track record of accomplishments.

Under his leadership, the Jeep brand has gone through a profound transformation and an unprecedented period of development, from a few hundred thousand units a year to several years of record sales, passing the million-vehicle mark in each of the past four years, becoming not only one of the fastest growing brands in the world but also the most profitable in the company.

Mike has held roles of increasing responsibility over the years, accumulating extensive leadership experience across all of our operating regions globally, achieving remarkable results in every position and always demonstrating determination in achieving his vision.

I am sure you will all provide your full support to Mike, so that we can work with him and the leadership team to execute our 2018-22 business plan with the same commitment and integrity that have already carried us so far.