John Collins believes Celtic's European struggles are down to the weakness of the Scottish Premiership.
The Bhoys were crushed 6-1 by Barcelona in their final Champions League pool game at the Camp Nou and ended their campaign at the base of Group H after collecting three points - registered in a home win over Ajax - from their six matches.
And ex-Celtic midfielder Collins told Sky Sports that while the Hoops were not good enough against the Catalan giants, they were hamstrung by not experiencing enough of a test domestically.
After a game in which Barcelona made 754 successful passes and Neil Lennon's men just 218, Collins said: "It was painful viewing for Celtic fans and a footballing lesson from the masters.
"In possession Barcelona didn't let the ball leave the grass, out of possession they pressed together and they absolutely destroyed Celtic, who lost confidence and became disjointed after the first goal went in.
"But the problem Lennon has got is that his team won 7-0 and 5-0 in the previous two domestic games (against Hearts and Motherwell) because their opponents were petrified and didn't press them, so when they were pressed by the world's best they couldn't cope with it."
Collins - whose former club top the Scottish Premiership by seven points from Inverness Caledonian Thistle after 14 games - added: "Celtic's passing was atrocious, too, while I don't think there was a tackle all night and they were a yard short all over the pitch.
"It was probably the longest 90 minutes those players have had to play in, and even though Georgios Samaras scored he doesn't look bright, sharp or alert; he holds on to the ball too long; and too often his head is down.
"I think it is now time for Celtic to buy a special player rather than three or four average-to-good players, but they will find it tough as there are no challengers for them in the Scottish Premiership."
Click here to watch a selection of reaction, views and goals from Matchday 6 in the Champions League.