Celtic will take on Old Firm rivals Rangers at Ibrox in Glasgow in the semi-finals of the Scottish Cup on Sunday afternoon.
McManus, who started his professional club football career at Celtic, believes that the 25-year-old former Wolverhampton Wanderers man will have a major say on the final result against Rangers.
“Playing for Celtic you get a lot of chances and sometimes that can play on your mind when you are not scoring," McManus told Sky Sports about Griffiths.
"But the most important thing is he still in there, and he's going to be a big, big player for Celtic at the weekend.”
Griffiths has scored 37 goals in all competitions so far this season, and he will be looking to hit the 40-goal mark as soon as possible.
The Scotsman is the best striker in Scotland at the moment, and he will be a nuisance to the Rangers defenders on Sunday afternoon.
Celtic will head into the match against Rangers as favourites, but Mark Warburton’s side’s chances of winning cannot be written off just yet.