Griffiths has scored 28 goals this season on loan from Wolves, one of which was in the 1-0 Clydesdale Bank Premier League win over the Hoops at Easter Road last December.
The 22-year-old was this week included in the Scotland squad for the World Cup qualifier in Croatia in June but McPake, who has taken an infection to try to ensure his recovery from a back injury for Sunday, believes he could first help provide a shock cup final win for the Leith club.
The former Livingston and Coventry defender said: "We are the underdogs. Celtic are the champions, they won the league at a canter, they had a great European campaign and they are a top team with top players.
"But on the other hand, we have the best striker in the league and I have said that all season.
"I have no doubt in my head that Celtic will be as fearful of Leigh Griffiths as we are of any of their players.
"I don't know if we need them to have an off-day but we have to be at our best."
Asked if there was an obsession about the Scottish Cup at Easter Road, given its absence from Easter Road since 1902, McPake, who joined Hibs on a permanent deal last summer from Coventry after a loan-spell, said: "I think there is, yes. I didn't realise it until I came to the club."