Then we killed the last of the previously-extinct Mursaat a few years later in Eye of the North - after finding out that at least some White Mantle survived (the Mursaat was explicitly stated to only have survived because it had put part of himself inside Justiciar Naveed). We wiped them out as well.
Then we find out a few years after that that we'd only slightly discomfitted the White Mantle and that they somehow had plenty of Mursaat constructs hanging around, giving orders. We wipe them all out again (The Battle of Lion's Arch).
Then, of course, in the missing parents Personal Story arc for Humans, we find out the White Mantle are still around. I therefore see no reason why the Mursaat won't show up again, like *roaches (Seriously? The first part of the word gets censored? The filter really needs some fine tuning and to stop looking at substrings :/).
It has been stated several times that Lazarus the Dire did NOT die completely.
The part in Naveed is most likely not the only part.
Also keep in mind his last words in that quest:
"Accursed human! You have done this to me! I will not forget this!"
"Countless generations will suffer for your actions this day!"