After signing for the champions in August, the Congolese midfielder lost 2-1 at former club Kilmarnock on his debut a fortnight ago and followed that by being on the losing side against Red Bull Salzburg in Austria in the Europa League on Thursday night.
With club captain Scott Brown still sidelined due to a hamstring problem, Mulumbu is aiming to be on the winning side in Perth.
“I’ve been frustrated because I haven’t won yet with Celtic – which is a bit embarrassing – but I’m just enjoying playing with the lads and, when the gaffer needs me, I have to be ready,” said the 31-year-old.
“The fans have been brilliant and the club is amazing,” he added. “The only thing that’s missing is winning because they’re used to winning all the time – until I signed!
“I don’t want to be a bad sign or bring the team bad luck but nobody has made that connection – yet! I’m just frustrated. I’m trying to do my best and hoping it comes soon. But there are no worries in the dressing room, no crises or stuff like that.
“We know what the trouble is and we know what we’re going to do. The quality of the players in the dressing room – no-one else has it in Scotland. We just need to fix those small details.
“When you’re used to winning, sometimes you forget about the details which can make the difference and, when you don’t do them, you don’t win. Once we fix those details and once we get our confidence back, everything’s going to be okay.”
Mulumbu made his European debut on Thursday evening but, while conceding that Salzburg are more than useful, he insists that the scoreline did not provide a true reflection of the play.
“We’re a better team than we showed,” he said. “We played out from the back a few times and got through our opponent but, with all the views we heard about Salzburg, I feel we respected them too much.
“When you score after five minutes and you have some chances to score the second we shouldn’t have dropped like that.
“The score definitely flattered them. Don’t get me wrong, they are a good team but we didn’t need to lose 3-1.
“If we had been braver on the ball we could have hurt them. I’m 100 per cent sure of that. Will we beat them in Glasgow? I think we will. I don’t think they will come with the same confidence because they know what Celtic is about.”
There is no disputing the fact that Celtic have lost their swagger and other teams no longer fear them as they did. Mulumbu accepts that recent displays have been lacklustre but remains convinced that trophies will be won this season. “Of course; if not, I’ll be in trouble!” he said. “Seriously, for sure. We have to win and we will. Celtic are used to winning all the time but since I signed, I haven’t won a game.
“We sense [the flatness of the displays] and it’s a direct comparison to how the team has done in the last two seasons; if we don’t have those seasons before and we win the league then everybody will be happy.
“The most important thing is the final result and, if we win the league, we will be happy regardless if it is flat or not.
“We just need to win the league and that starts this Sunday by winning against St Johnstone.”