Griffiths won a trio of player of the year awards after scoring 40 goals last season but has started only 10 league games this campaign.
He missed Sunday's win over Hearts with a calf injury and Rodgers made it clear the striker must improve his fitness.
"What I've said to him is about making sure the talent is a working talent," said the Celtic manager.
"Because talent's not good enough on its own.
"You have to work at it - and it's a cycle. It's one where, when he's playing and fit, Leigh's very good but can still work harder in training.
"The problem is if you're not getting the games and then if you're not quite on it in training then you're losing fitness.
"But then the moment you're asked to come in and get up to speed in a real high-tempo, intense game, the moments that are there for you to risk injury are far greater."
Rodgers named Dedryck Boyata and Callum McGregor as examples of players who have shown great professionalism to work hard and force their way into his plans.
"You have to work tirelessly every day or unfortunately you'll get left behind," he said.
Rodgers echoed the words of his predecessor, Ronny Deila, who spoke of the need to be a "24-hour athlete".
"It's a difficult period for him with that injury," the Celtic manager said of Griffiths, who has lost his place to summer signing Moussa Dembele.
"But the only way you can overcome that is by working 24 hours a day for your profession, for your life.
"He's a good kid, a great boy, but he understands where he's at. We all know the talent, but it won't be good enough on its own.
"We speak a lot and it's not easy because he hasn't played maybe as much as he did last year. The reasons are simple for that.
"He's a player I really admire, a player we really want to have here.
"He's part of the squad, but I want him fit and available. He knows that and it's something he's working harder now to try to achieve."

Four hundred devices have been purchased, at a cost of Â£1.5m.
Body Worn Video Technology, as it's known, is already in place in Derry City and Strabane District.
The cameras are clipped on to the top half of an officer's uniform and can provide eight hours of continuous recording.
Officers have been trained to tell people when they are being recorded.
Supporters of the scheme say the cameras will improve the quality of evidence that police officers can gather.
They will be particularly useful in assisting prosecutions in cases of domestic abuse, for example.
The roll-out follows on from a pilot scheme in Foyle in 2014-15.
Body cameras are routine in other parts of the UK and were first introduced about a decade ago.
Studies have found that they dramatically reduced complaints against officers by members of the public, by up to 93% in one case.
PSNI Chief Supt Chris Noble said: "Video evidence puts the victims of crime first.
"The pilot of this technology in Foyle district evidenced how Body Worn Video has the potential to improve the quality of evidence provided by police officers. It also increases the number of offenders brought to justice.
"The introduction of this new technology is the latest example of our commitment to these principles as we continue to work together with the community to keep people safe."

Forty-two refugees, belonging to five families, were greeted on arrival by the Uruguayan President, Jose Mujica.
They will spend two months in accommodation near the capital Montevideo where they will learn Spanish and  attend classes on local customs.
Other Latin American countries have taken Syrians in but Uruguay is the first to assume all resettlement costs.
Officials say the two-year resettlement programme will cost Uruguay around $3m (Â£1.9m).
They say the adults have already been guaranteed  jobs and the children have places in local school. A second group is due to arrive next year.
Brendan Rodgers has urged Leigh Griffiths to work harder if he is to force his way back into Celtic's team.
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Police officers in Belfast will wear body cameras while on duty from Wednesday.
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A small number of Syrian refugees have arrived in Uruguay from Lebanon.