After scoring 24 goals in six successive wins, they came through a relentless encounter in Holland on Thursday with a 2-1 defeat which saw them into the Europa League group stage.

Another major test awaits on Sunday against the champions but Postecoglou stressed the game would not decide anything.

"It's August," he said. "I know people like putting defining moments in at all different times. There will be defining moments all throughout the season.

"What's important is we have had a really positive month so far and in terms of improvement in our performances and getting reward for that. Sunday is another opportunity to build on that momentum for us.

"It's the end of August and there is still a hell of a lot of football to be played. And from our perspective, either way, we are not going to get too carried away with whatever the outcome is.

"We want to continually improve our performances and Sunday is a great chance for us to establish our football against a very good opponent in a difficult environment. What a great test it's going to be for us."

Croatia right-back Josip Juranovic could make his debut while James McCarthy and James Forrest are pushing for fitness after missing the Dutch trip.

Postecoglou added: "Out of last night, obviously Greg Taylor has got his shoulder problem. It looks like it is nothing too significant so there is still a chance for Sunday.

"Kyogo (Furuhashi) got his usual bruises and knocks. He is getting tested out physically every game, I think people are figuring out that he is one of the ones they need to stop and try to find, shall I say, creative ways to do that. But he usually bounces back pretty well so I am hopeful he will be fine."

Celtic earlier announced they had agreed a deal to sign 23-year-old defender Liam Scales from Shamrock Rovers as the transfer window activity steps up further ahead of Tuesday's deadline.

"It's one we have been working on for a while obviously," Postecoglou said.

"His club has been involved in European competition and been doing really well and were keen to hold on to him for as long as he could. That was okay with us but good to get him in.

"He's a good young talent, can play a couple of positions at the back, gives us some extra reinforcement and depth in an area we need it.

"We still have a few days to go so I expect there to still be some activity. There could be ins and outs, we will wait and see, but we are still working on a couple of things.

"Obviously as the timeline gets tighter things get a little bit more challenging but hopefully we will still have some activity between now and the end of the window.

"What's going to happen in the next few days, to be fair I haven't had a lot of clarity in this last month or so, things have been a bit murky with everything that's been happening. I don't expect to be too clear cut what happens over the next few days.

"We have just got to be agile and alert and when things happen be ready to act on them in terms of players coming in and going out.

"We want to be in a strong position at the end of this transfer window. It's been a hectic time for us but I think we have done a pretty good job so far and we really want to come out of this window to be in the strongest possible position."