The Brazil international moved to the French champions from Chelsea in the off-season, for a record-breaking £50 million fee.
The 27-year-old has made 20 appearances in all competitions thus far, but has come in for some criticism after some shaky performances.
Juninho, who won seven consecutive Ligue 1 titles with Lyon during the last decade, has come to the defence of his compatriot.
"David Luiz has not started his season as he should," the 39-year-old told RMC.
"Today, he still has not the form expected of him. I would not say it is bad.
"I watch PSG play. He commits mistakes from time to time but all the goals are not because of him.
"Maybe he should play Marquinhos. But this team does not live together.
"They are not able to play all matches at the same level. I do not know what happens."
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