Barton was in impressive form for his club Newcastle United last season, recording nine Premier League assists and scoring four goals. He is, however, set to leave the Magpies after a publicised fall-out with the club's hierarchy.
Capello rates Barton, who earned an England cap in 2007, but did not select him in his squad to face the Netherlands for Wednesday's friendly after revealing his fears over the 28-year-old's potential for ill-discipline.
"He is a good player but he is a dangerous player because sometimes you could end up playing 10 v 11. He might get sent off," Capello said in the Guardian.
Asked if he was worried Barton's "head might go", the Italian said: "Yes. But this is not a good moment to decide something about him. When a player is fighting with his club, it is normal to stay outside. The relationship between a club and a player is really important."
Barton later commented on the subject via his Twitter account: "Am goin [sic] to take the fact he(Fabio C) used the words me and good player in the same sentence, all I want is the opportunity to prove the rest.
"My goal is to get in that Euro2012 squad and team for the Olympics, all I can do is more of what I did last season, if its [sic] enough its [sic] enough.
"To even be mentioned in the reckoning, is a testament to how far i've [sic] come since my troubled yrs [sic], looking forward to pushing for a place...."