The Brazilian netted twice in two minutes before Juve hit back through Arturo Vidal and Fabio Quagliarella, who equalised with 10 minutes left.
"We are disappointed and a little bit deflated," said Di Matteo.
"Oscar did a perfect job tactically and scored two goals."
The 21-year-old midfielder, who joined from Brazilian side Internacional for about £25m in the summer, opened his Chelsea account with a deflected shot, before curling in a stunning second.
Di Matteo, who led Chelsea to their first Champions League title last season, added: "It was a great debut for Oscar.
"It was just the right moment for him to start the game in the Champions League. He has been mostly away with his national team so we need a bit of time to work with him. He was perfect tonight.
"He took on board the information we gave him and it was great to see we have a player like this in our ranks. He is a big talent. He has got a bit of everything.
"We shouldn't forget he is still a young man but he is already well established with the Brazil team. We are lucky to have him here."
But the Italian was disappointed by his side's failure to defend the lead Oscar had given them.
"I think we should have dealt with it a little bit better," he said of the late equaliser.
"We lost possession just beforehand and we should have reacted much quicker to losing the ball than we did."
Defensive midfielder John Obi Mikel surrendered the ball in the build-up to the equaliser, and compounded his error by failing to stop Claudio Marchisio from playing in Quagliarella.
"If I want to make up an excuse, I can," the Nigeria international said. "But, for me, I gave the ball away. I put my hands up, we move on.
"I'm sorry about the mistake, but there are no excuses. I think that's what you get from playing in the Champions League.
"You make a mistake, you get punished for it."
The Blues next travel to play Danish side FC Nordsjaelland, beaten 2-0 by Shakhtar Donetsk in their Group E opener, for their next Champions League game on Tuesday 2 October.