"I don't know what's going on at club level with the guy but he's a talented boy," the Ireland assistant coach told reporters in France.
"But it does happen, players have dips in form. He's still an absolutely dangerous player and he showed a bit of better form before the end of the season for Chelsea. We'll have to keep an eye on him."
The 44-year-old former Manchester United captain has also been impressed by Belgium and United midfielder Marouane Fellaini, who made his bow at the European Championship in Belgium's 2-0 loss to Italy on Monday.
"I like him. I thought the criticism he had when he went to United was a bit over the top," Keane added.
"I saw him a few times live when United weren't great and thought he did okay, but he seemed to be the scapegoat for the fans. We saw him recently scoring some important goals."
Ireland, who opened their campaign with a 1-1 draw with Sweden, face Italy in their final group stage on June 22.
(Reporting by Ian Rodricks in Bengaluru)
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