"A few of the boys would admit they took their foot off the pedal, not taking the mickey or anything, but getting away with a few things."
The 32-year-old is acting Gillingham captain while skipper Barry Fuller recovers from a knee injury.
"The new gaffer won't take any drop in effort from us, and that's what you need in a team that's pushing for promotion," said Bentley.
Hessenthaler took over as Gills boss after Mark Stimson's sacking following the club's relegation from League One.
Bentley was speaking after Gillingham's 2-1 home defeat in a friendly with SPL Hearts, in which he was asked to fill in at centre back.
"We passed the ball really well, and the manager's asked us to press sides on the ball, and we didn't give them any time. We were very solid, especially in the first half when we were probably the best side."
Gillingham opened the scoring with a Luke Rooney penalty just after half-time - Arvydas Novikovas and Ismael Bouzid netted for Hearts.
New signing Adebayo Akinfenwa made his Gills debut in the game, coming on as a substitute in the second half.
Source: BBC Sport
Source: BBC Sport