The Cs finished off the Indiana Pacers with a 105-102 victory in Game 4 on Monday ... and when Grousbeck received the Eastern Conference Finals championship trophy, he acknowledged the tragic loss that was announced hours prior to the matchup.

"All I can think about right now is we lost a great Celtic today," the entrepreneur said during the ceremony. "Bill Walton, one of the greatest Celtics of all time, this is dedicated to Bill."

As we previously reported, the 71-year-old Hall of Famer died surrounded by his loved ones this week after a battle with cancer ... which came as a shock to many, as he never showed signs of an ongoing health struggle.

Tributes have since poured in ... with the entire NBA community, as well as former President Barack Obama, sending kind words about his successes on and off the court.

Walton was also a massive Grateful Dead fan ... having attended more than 1,000 shows over the course of his career. Dead & Company -- a band consisting of former GD members -- also shared a touching message ... saying, "Fare you well, fare you well, we love you more than words can tell."

"Bill was an irreplaceable force and spirit in our family. Father Time, Rhythm Devil, biggest deadhead ever. Over 1000 shows and couldn’t get enough. He loved this band and we loved him."