The midfielder was a teammate of the Frenchman at Newcastle and knows first hand what he is capable of when the former Paris Saint Germain player is on his game.
"He is exceptional and has the best left-foot I have ever seen," said JJ of the 30-year-old who made the switch to Fratton Park over the close season. "He can be unplayable on his day and we have to hope he isn't on it.
"His left foot is such that he can plant the ball on peoples' heads. He was great for Newcastle when I was up there, but hasn't reached that level at Portsmouth as of yet. He is one of those players you've got to watch.
"He is a great player and if Harry can get him going he is going to be very special for them."
Another of the same ilk is also a former teammate from JJ's Newcastle days - the unpredictable Lomana Lua Lua.
"Lomana has a God-given talent," added JJ. "I think he came into the game pretty late, he loves his skills, he is sharp and exceptionally strong - more than people realise. He is another danger."
Jermaine Jenas warns to beware of the left foot of Pompey winger Laurent Robert.