Three goals in a 14-minute spell in the first-half left Celtic reeling, while a red card for Charlie Mulgrew midway through the second period, when he conceded a penalty, added to their agony and leaves Lennon with much to ponder 10 days before their opening tie against HJK Helsinki.
The decision to play with three at the back – Thomas Rogne, Victor Wanyama and Mulgrew – backfired in dramatic style as the Eredivise champions exploited a glaring lack of cohesion and resistance.
The gradual improvement shown in the tour games in Germany was sent into reverse in the Amsterdam ArenA, where they failed to register a shot of any note. Adam Matthews and Emilio Izaguirre made their first appearances after injury and illness, and were deployed in wide midfield positions. The system gave Celtic a foothold in the match, and they looked comfortable, even though forced to cede possession and territory to a team with pace and guile.
However, after 13 minutes, Siem de Jong changed the complexion of the match with a 23-yard shot which flew across Forster and high into his net. Eight minutes later, serious questions were asked of their marking as Viktor Fischer found space in the box and, with a quick turn and simple pass across the six-yard line, stranded Rogne and Forster, allowing Kolbein Sigthorsson to tap home.
If the floundering Rogne was exposed then, much worse followed six minutes later when he lost possession while attempting to carry the ball out of defence allowing Thulani Serero to slip it beneath the advancing Forster.
Lennon's answer was to revert to a flat back four, with Wanyama pushed into midfield, but the damage was done and limiting it was all that was left. They appeared to have stemmed the incessant flow towards their goal until the 64th minute, but Mulgrew's dismissal for stopping a Fischer shot on the line with his arm, and Danny Blind's converted penalty, underlined the gulf between the teams, and how much work Celtic still have to do, with only the Norwich and Inter Milan friendlies in which to do it.