The show's original star Andy Whitfield was forced to leave the series last year to receive treatment for a recurrence of non-Hodgkin lymphoma.
Earlier this week, it emerged that Liam McIntyre has now signed up to take on the role.
"This was not a decision we made lightly," Lawless told the New York Post. "It hurt us to do it and Liam has some terribly big shoes to fill. We don't expect him to be Andy, no-one could, but he's going to be Liam. And we believe he's the right guy to reimagine that role and keep this series on fire."
Lawless revealed that the casting directors saw "over 1000 people all over the world" but explained that McIntyre was the "best actor".
However, she added: "We've got to keep the love going for Andy. He is putting himself and his family first so he'll live to fight another day!"
Filming for the second season of Spartacus is expected to begin in April.