Having conceded three goals at home in Thursday's round of 32 first leg, Deila claimed Celtic will need to produce "the best performance...under my command" in Milan to advance.
But the Norwegian manager was thrilled with how Celtic came from 2-0 down to equalise and, after again going behind, levelled again in second-half stoppage time.
Celtic substitute John Guidetti produced the decisive moment, chesting down Liam Henderson's chipped pass before volleying the ball into the roof of the net in the 93rd minute.
"It's going to very, very tough now," Deila said.
"We have to be lucky and put together the best performance so far under my command. But it's possible, so we are looking forward to the game.
"I believe we can score in the San Siro. The most important thing is not to concede as we did tonight. Defensively, we have to be much better.
"I did not have good thoughts at 2-0 down, I have to admit that. It was a very tough start. We actually started well and had them under pressure, but then it was just too easy for them to score. I was very worried but we showed a good mentality to get back into the game.
"It was an unbelievable football match with an unbelievable atmosphere. I'm so proud of the boys today. At 2-0 down, it's so hard to get back, especially against Italian teams.
"We talked earlier in the week about being brave, wanting the ball and trying to create things that's why I'm very proud today. We caused them problems but it's a pity we gave them two easy goals."
Deila defended Celtic goalkeeper Craig Gordon, who effectively gifted Inter their third goal just before half-time by dropping the ball at Rodrigo Palacio's feet.
And the 39-year-old hailed Guidetti's first goal in 14 matches.
"John Guidetti has been in good focus lately. He has trained well and when you do that you get results," Deila said.
"It was a good performance when he came on and a good goal as well."
