The Gorgie side beat Ange Postecoglou's men 2-1 at Tynecastle Park in the first game of the season.
Defeat added more pressure on the Australian following the Champions League qualifying exit to FC Midtjylland.
Neilson watched Celtic defeat Jablonec on Thursday in the Europa League qualifier and “thought they were very good”. In terms of change, however, he believes there is “not a lot” despite the upturn in results.
"They’ve got the striker Kyogo Furuhashi who has come in and given them something different,” he said.
“But I would expect it to be much the same team, maybe only Tom Rogic coming in for Soro with Callum McGregor dropping back.
“So there’s not a lot of change from Celtic, I wouldn’t think.
“They’ve got more momentum – but that’s football.
"Fifteen days ago it was a different story. You win a couple of games and it all changes. But it’s the same here. You are only as good as your last game.
"We have had a good start to the season, six wins on the bounce, but the game on Sunday will be the biggest challenge we have had yet.”
League Cup and Celtic Park hoodoo
In Hearts’ last outing they won their first league fixture at St Mirren since 2010. You have to go back even longer for the last win at Parkhead. A Michael Stewart penalty handed the Jam Tarts a 1-0 victory in the League Cup in 2009.
If the team are going to end their League Cup hex then they will have to overcome another away day hoodoo.
Hearts have not won the competition since 1962, while reaching just two finals since then.
"Obviously you want to go and win cups as well as doing well in the league," Neilson said. “This is a great opportunity for us.
“We had a good result against Celtic a couple of weeks ago. But it’s a different issue when you go to Parkhead. It will be tough.
"But we go there looking to win – and to go as far as we can in the cups.
"For us, we’ve got a good record at home. It’s the away games we need to win if we’re going to push ourselves up the top end of the league and compete in cups.
“We knew that was an important game for us last week [winning 2-1 at St Mirren]. There’s no point in beating one of the Old Firm and not following it up, so it was good to do that.
"Obviously we’ve now got the Premier Cup on Sunday. To get into finals and win the cup, we have to go and beat Celtic.
Neilson added: “I was at Celtic Park on Thursday night and there were 50-odd thousand there, which was brilliant.
“It was one of the motivations when we were in the Championship, getting back to these big games in front of big crowds, whether that be at Tynecastle or elsewhere.
“We’ve got a number of players who have played in these big environments . They’re all looking forward to getting back to it.”