The Ireland international came off the bench to score on his Champions League debut in the 5-1 win over Slovan Bratislava on Wednesday.
And the 23-year-old struck twice in quick succession to turn Sunday’s Premier Sports Cup quarter-final against Falkirk on its head and send Celtic on the way to a Hampden meeting with Aberdeen.
It was Idah’s first 90 minutes of the season and he feels there is room for improvement.
“I was delighted with the two goals, but I was probably still a bit annoyed with my performance in the end,” he said.
“I thought I probably could have got more goals, that’s the frustrating thing.
“I know I got two goals, but there are other things that I’m not really happy about, so it’s a bit of strange moment for me right now.”
Idah had an uncertain summer amid transfer speculation and the arrival of new Norwich manager Johannes Hoff Thorup before sealing his permanent move to Celtic Park in mid-August.
And he feels he will get better when he gets fitter.
“One hundred percent,” he said. “Look, I didn’t do much in pre-season. Before anyone jumps on that, that wasn’t me chucking it or anything, it was just how we trained down at Norwich.
“Obviously, it’s not as intense down there as it is here, we’re at a big club and we want to be on it.
“So when I first came I was probably a week behind and now I’m getting the minutes in I’m starting to get goals.
“I feel good now and I’m probably not far behind. I just need to keep sticking with it and I’ll get there.”
Idah equalised for Celtic with 20 minutes left before a Nicolas Kuhn-inspired comeback ultimately resulted in a 5-2 win over William Hill Championship leaders Falkirk.
“It was a tough game,” Idah said. “To be fair to them, they came at us really sharp and quick and probably blew us away a bit.
“It was disappointing in the first half because it was probably the worst start we’ve had this season and for a while, it wasn’t great. But we stuck at it and we stuck together. We spoke at half-time, came back and showed what we were made of.
“We played Champions League the other day and we were on such a high and it was always going to be tough. We changed most of the squad around, not many of us have played together. So it was a difficult first half, but we got it together in the end.”