Frank Lampard has accused Ross Barkley of showing a "lack of professionalism" after he was pictured shirtless in a nightclub during the international break.
It was the second time the England international had courted negative headlines in recent months after he was involved in an incident with a taxi driver earlier this season.
Barkley has also been suffering with an ankle injury over the last few weeks, and Lampard says he must knuckle down after leaving him out of the squad for Wednesday's Champions League trip to Valencia.
The head coach said: "There are two separate issues; one is the headlines this week. Ross knows how I felt about the first headline. I backed him then and I still back him now. But he showed a lack of professionalism as far as I'm concerned.
"It's not something that I want to come down hard on, because I understand all my players are humans and have lives. I don't police every moment of their lives. So I don't think it's a terrible thing that he's done.
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"But Ross has had an injury, he's still kind of carrying an injury, so he just has to work his way back in and train well.
"I certainly don't hold anything against my players. I like Ross and he needs now to show all his reactions on the pitch."
'Chelsea one of world's best teams'
Valencia surprised Chelsea by beating them 1-0 at Stamford Bridge in September, and both sides are level on seven points with Ajax at the top of Group H.
But the Spanish side's boss Albert Celades warned before the return match at the Mestalla: "Chelsea are a super team, one of the best in Europe and are playing very well at the moment."
The Blues have won nine of their 13 games in all competitions since losing at home to Valencia, and their captain Dani Parejo added: "They are one of the best teams in the world because of their name, their history and everything else.
"They are in fantastic form even though they lost to Manchester City the other day. But we have to focus on us. We have shown we can compete against any team, whether it's in Spain or in Europe.
"We don't need to motivate ourselves at all for this one given who we are up against and what is at stake."
Super 6: Blues back to winning ways?
Chelsea face a stern test in Valencia, before welcoming West Ham in Saturday's Super 6 round. More than 95% of players are backing Frank Lampard's team to get back to winning ways in the Premier League, but who will you tip, with £250k on the line? Play here for free.
