The out-of-sorts France captain has hardly featured for Barca in recent weeks and was whistled off the Stade de France pitch by the home fans when he was substituted in Wednesday's 2-0 friendly defeat by Spain.
"If he's not playing, it's maybe because, among other things, there are other players more inspired than him," Guardiola said of the 32-year-old Henry at a news conference.
"I need him to have the same ambition that made him the best over many years.
"He knows he has to improve, although I believe he is better than he himself thinks. I am sure he'll give us the best of himself over the next two months."
Guardiola said it was not true that Henry was saving himself for the World Cup in South Africa and pledged to help the player get over the slump.
"He knows that he's not on his best form and that's why he says it's logical that other players play," he said.
"He's paying the price of having been the best and won everything. He is not happy with the situation and I am here to help him."
Former Arsenal player Henry was a key figure in Barca's treble-winning campaign last season, scoring 26 goals in all competitions, but has started only 13 of 24 league matches this term and scored just three times.
