Wing-back James rifled home the Blues' final goal in an emphatic 3-0 Stamford Bridge success over the Serie A champions, having set up Pierre-Emerick Aubameyang's strike following Wesley Fofana's first-half opener.
A fine evening for the hosts was marred only by £72million summer signing Fofana limping off due to a first-half knee injury which has left head coach Potter "concerned".
Yet the former Brighton boss was full of praise for England international James, claiming he could become a club "legend", after Chelsea got their European challenge back on track following just a single point from their opening two Group E fixtures.
"I think Reece is just a fantastic player and he's a young player, so he's developing all the time," said Potter.
"He'll get better and better as we go. I've really enjoyed working with him, he's got a lot to offer.
"He can play in any game in the world, he's just tremendous. My job is to try and help him reach his potential because his potential is beyond the sky.
"For us, we love him. He's so important for us.
"He could be a Chelsea legend, that's the level that he has, the potential that he has.
"But for that to happen he has to be part of a successful team and that's what we're working towards."
Centre-back Fofana claimed his first Chelsea goal by finishing following a goalmouth scramble with 24 minutes on the clock in west London.
But the French defender soon departed and later left the ground on crutches and wearing a knee brace, leaving Potter with an anxious wait to discover the extent of the issue.
Aubameyang's second goal in two games following James' fine right-wing cross put the Blues in complete control early in the second period before the latter put the game beyond the Italian visitors with a thunderbolt into the roof of the net.