The Greek keeper will join AFC Bournemouth for the remainder of the campaign for a loan fee of €1.5million, it is understood.
There is also an option to buy clause included in the deal worth £16m, but that will only be activated should Mandas start a set percentage of games between now and the end of the campaign.
If that option is not taken by the Cherries, there will be an extra €1m loan fee payable to the Italian club.
Mandas is one of two players set for medicals at the Cherries today, with Brazilian winger Rayan set to undergo his tests as he finalises a move to the south coast.
The keeper has made one appearance for Lazio this season, playing in a 1-0 victory over AC Milan in the Italian cup.
He has been an unused substitute in every Serie A game this term, with Ivan Provedel the first choice keeper in the league.
Mandas will become the second keeper signed by the Cherries this January, after Fraser Forster joined on a six-month deal after Will Dennis picked up an injury.