His recovery is a major boost for Rafa Benitez, who recommended the club use an Italian surgeon he knew from his time managing Napoli.
Rome-based Professor Pier Paolo Mariani performed the operation on Lejeune after he damaged his ACL in July.
Such an injury usually means a spell of between nine and 12 months on the sidelines and Benitez reacted by signing centre-back Federico Fernandez for £6m from Swansea.
But the Magpies boss believes Mariani has shaved four months off the period of absence and rates him as the best knee surgeon in Europe.
It is understood he can perform the operation to repair the damage in just 20 minutes - a third of the time of other surgeons - and so reduces the post-procedure swelling, meaning Lejeune was able to walk without aid almost immediately.