Vunipola played his part in every Lions Test against Australia in the summer, coming off the bench in the first and third Tests against the Qantas Wallabies but starting the second clash in Melbourne.
And Lancaster is adamant the Saracens prop has what it takes to make an impact from the get-go on Saturday.
"We have prepared for being without Joe and we have no doubt Mako will slot straight in and perform," said Lancaster.
"It is important that Joe is with his partner at this exciting time. Family must always come first and everyone in the squad wishes them well."