Spader - who plays the eccentric Robert California - will depart the NBC comedy in the eighth season finale.
"We always kind of thought of this as a transition year," Lieberstein told Vulture. "It was just kind of fun, living in the moment and just enjoying [Spader's] presence, and not really planning what it was going to be like in seasons to come."
The actor and executive producer praised Spader's performance, calling California a "bizarre, fascinating character".
"I thought we found a lot of interesting things to play with him; as his life changed, opened up, we learned about his marriage, his divorce," he said. "And I loved his relationship with Andy (Ed Helms) too."
While The Office is yet to be picked up for a ninth season, Lieberstein added that he is "optimistic" about the show's future.
"I see another year - a full year, and a great year," he said. "There are negotiations going on, but I don't think they feel different than other shows that have negotiations in their later years."
The Office continues on Thursdays at 9/8c on NBC.
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Watch James Spader in The Office below: