First-half goals from Mounir Obbadi and Radamel Falcao helped Ranieri's men to their seventh victory in 11 Ligue 1 games this season.
Despite sitting second and level on points with league leaders Paris Saint-Germain, the Italian is concerned by their efforts after the interval.
"During the first half the movement was good and then they stopped," Ranieri said.
"During the first half, everybody showed their ability and played the ball. At one point they all stopped.
"It is not a problem of shape. Physically they are all in very good shape. It is a mind problem, it is the big problem.
"However, we are on the top of the table, so it is normal. Sometimes it works straight away, sometimes it takes time."
Bafetimbi Gomis pulled a goal back for struggling Lyon on 62 minutes, but Monaco held on to claim three points.
Ranieri said his side needed to use star striker Falcao more often, lamenting their inability to get more out of the 27-year-old.
"Falcao is doing well always. He always works for the squad. I think that everybody should think a bit more about him on the pitch," he said.
"We should give him more the ball. I speak about good ones, because there were no good crosses or passes.
"It is important to know how Falcao is moving on the pitch, because he is working for the team. All the time he's doing the pressing – protecting the ball, so it is important to know him well."
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