Griffiths spent last season on loan at Lincoln City and this summer he joined Portsmouth on a season-long loan.
The 21-year-old goalkeeper has appeared in all 16 league games for Pompey this season and has managed to keep five clean sheets.
Cowley is of the opinion that the English Under-21 international is mature beyond his years and stressed that Griffiths has proven that he is an excellent goalkeeper.
The Portsmouth boss pointed out that until their last match at the weekend against Morecambe, which finished 1-1, his side were the team who boasted the record of “lowest expected goals conceded” against them in League One.
“Josh is a brilliant human being, so mature – maturity beyond his years”, Cowley was quoted as saying by The News.
“He’s been waiting to have something to do.
“Up until Morecambe we’ve got the lowest expected goal conceded in the division.
“But he’s shown he’s a top keeper.”
On Saturday, Portsmouth will take on Derby County, and West Brom will hope to see Griffiths continue his impressive form against Paul Warne’s side.
