Uruguay's all-time leading goalscorer Luis Suarez could only sit and watch from the sidelines as Costa Rica's shock 3-1 victory in Fortaleza.
The Liverpool forward is still recuperating from keyhole surgery on a knee injury and is being held back for next week's clash against England in Sao Paulo.
On this evidence his arrival cannot come too soon for his country as although fellow striker Edinson Cavani opened his World Cup account with a 24th-minute penalty he lacked the penetration and drive Suarez brings to the team.
With Uruguay missing that spark and failing to capitalise on their lead Costa Rica seized their chance with two goals in three minutes early in the second half to blow Group D wide open after just one game.
Arsenal's 21-year-old midfielder Joel Campbell, who hit the headlines with a stunning strike against Manchester United in the Champions League while on loan at Olympiacos, and Oscar Duarte's first international goal did the damage as Suarez remained rooted to the bench.
Campbell then applied the coup de grace six minutes from time with a slide-rule pass which substitute Marcos Urena tucked past goalkeeper Fernando Muslera from a narrow angle with virtually his first touch.
Uruguay were supposed to be the ones with the goals in reserve as they started with Cavani and Forlan, who had 57 between them even before the latter scored his 22nd in 63 internationals.
Suarez beats them all with 39 but it is not only his goals but the threat he poses and the runs he makes which usually give his nation the edge.
Without him Uruguay looked lifeless and lacking in ideas and that only served to give their opponents the confidence to believe they could get something out of the game and they duly did.
Maxi Pereira's wild swing at Campbell in added time summed up Uruguay's frustration and the 30-year-old's red card rules him out of the England game.
The match kicked off with pitchside thermometers showing 33 degrees celsius but the temperature was raised further when Oscar Duarte's foul on Diego Lugano early on ensured a Uruguay handball in this World Cup passed without major incident - unlike Suarez's save on the line against Ghana which helped put them into the semi-finals in South Africa four years ago.
Lugano's centre-back partner Diego Godin stabbed home at the other end, only to be denied by an offside flag - but there was no excuse for Cavani uncharacteristically slicing wildly wide with the goal at his mercy seconds later.
Yeltsin Tejeda, guilty of several reckless tackles in the first half, escaped sanction for a two-footed lunge on Cristian Rodriguez but he was punished from the resulting free-kick as Julian Diaz's challenge on Lugano in the penalty area was more like a rugby tackle, allowing Cavani to slot home from the spot.
Costa Rica's response saw Campbell's audacious left-footed effort from 30 yards whizz just past Muslera's left-hand post and Christian Bolanos fire into the side-netting via a deflection.
Muslera also missed Rodriguez's inswinging corner but, fortunately for him, so did Giancarlo Gonzalez at the far post while at the other end Forlan's deflected shot looped up wickedly and Keylor Navas only just got back to tip it over the crossbar.
For all Uruguay's supposed potency up front they were made to pay for not providing their feared strikers with enough chances as Costa Rica turned the match around in four second-half minutes.
Wing-back Cristian Gamboa's cross picked out Campbell 12 yards out and he hammered the ball unerringly past Muslera, who was helpless to keep out a diving header by Duarte who, moments earlier had missed a glorious chance for an equaliser from a carbon-copy Bolanos free-kick move.
Coach Oscar Tabarez sent on midfielders Nicolas Lodeiro and Alvaro Gonzalez to rectify the situation - Suarez's fitness meaning even this was not a dire enough situation to risk him - but it was Costa Rica substitute Urena who had the decisive say before Pereira lost his head late on.