Michael Rosenbaum will reprise his role as the villainous Lex in the CW drama's final instalment, due to air on May 13.
"With Michael being back, there was a lot of anticipation [and] a lot of nervousness on everyone's part," Welling admitted in an official CW interview. "Then he shows up... and it's like he never left. I realised, I think, how much I missed him."
The actor suggested that Rosenbaum's presence is vital to the finale, claiming that "there is no Superman without Lex".
"I think for Clark's journey, Lex definitely had to be a part of it," he said. "It's been a few years now where I've been out there saying [that] there is no Superman without the Clark and Lex closure."
Rosenbaum previously insisted that Lex's final scenes will wrap up the series in "the right way".
"I swear to God, I did it for the fans," he said. "I never want to hear, 'Why didn't you go back?' I wanted to end it the way it should have."
The two-hour Smallville series finale airs tomorrow at 8/7c on The CW.
Watch Tom Welling discuss the Smallville series finale below: