The Bhoys smashed five goals past Kilmarnock in the Scottish Premiership on Saturday afternoon, but former Hoop Collins was not completely impressed by Neil Lennon's men, for whom Georgios Samaras netted a hat-trick.
"The five goals will give Celtic and the fans confidence, but it will be a different ball game against players like Lionel Messi and Neymar," Collins said of the Glaswegians, who beat Barcelona in last season's Champions League group stages.
"They are going to be on the back foot and have to work hard, stay tight and keep themselves in the game - and not give the ball away cheaply as they did against Kilmarnock. This will be a proper test."
Collins - whose roll call of clubs also includes Hibernian, Everton and Fulham - believes Celtic began their fixture at Rugby Park slowly.
But he says their five-star victory was as much down to a tactical error from Kilmarnock as it was the Bhoys' attacking acumen.
"I think Celtic started poorly in the first half, but in the second half they controlled the game," added the one-time midfielder, who collected 58 caps for Scotland and won Ligue 1 with Monaco in 1997.
"Kilmarnock backed off and tried to be more compact but it backfired because they couldn't get up the park and were chasing the ball and getting tired.
"You thought it was only a matter of time before Celtic scored again and they were comfortable winners in the end."
Kris Commons and Amido Balde topped and tailed the scoring respectively with Samaras' treble sandwiched in between and Collins says the Greek forward was a constant menace to the Killy backline.
"Samaras had his purple patch and got his goals and in the second he was dangerous and penetrating," added the ex-Hibernian boss.
"He frustrates me because he doesn't always do the simple things well enough but he was a real threat in this game."