D-day for Wayne Rooney turned out to spell disaster for goalkeeper Robert Green.
Theo Walcott in 'B' action
As England fans waited for Manchester United to offer information about the all-important MRI scan on Rooney's broken foot, Sven-Goran Eriksson was hit with another injury blow.
Green was carried off with a suspected torn groin muscle, five minutes after coming on as a sub as the B team lost.
England had led in Reading's Madejski Stadium through Jermaine Jenas - but 10-man Belarus fought back well.
They equalised with a freak goal thanks to Green's injury and sub Sergei Kornilenko grabbed the winning goal nine minutes from time.
It was a heartbreaking moment for the Norwich man and presents a potential problem for Eriksson, who has a shortage of top-class keepers at his disposal.
There were positive moments for the England manager as Aaron Lennon and Theo Walcott showed flashes of their speed and flair.
Peter Crouch linked up well with Michael Owen in the strike partnership which could start the World Cup against Paraguay.
But there were also worrying aspects of the performance, particularly at the back where England looked sloppy.
Green looks certain to become the first casualty to drop out of the 23-man squad.
He came on at half-time for David James and immediately fumbled a free-kick which almost trickled into the corner of the net.
Then his left leg gave way as he took a goal-kick and he crashed to the turf in agony.
The ball rolled out to Vitaly Kutuzov, who stroked it into the empty net and levelled the game at 1-1.
Green was replaced by Liverpool's third choice keeper Scott Carson, who has played only 16 league games in his career.
Jermaine Jenas had put England ahead in the first half but Belarus grabbed a winner, nine minutes from time, Belarus played the last 17 minutes with 10 men after Sergei Omelyachuk was sent off for a second yellow card, for a foul on livewire Lennon.
The visitors hit the post on the stroke of half-time but England should have been awarded a second-half penalty.
Sergei Shtanyuk clearly handled to take the ball off Sol Campbell's head - but the offence went unpunished.
Carson, 20, spent the final three months of the season on loan at Sheffield Wednesday, fighting relegation to League One.
The Owls want him back in the Championship next season but first he looks like jetting off to the World Cup.
It would mean yet another uncapped player in Eriksson's squad.
He already has Lennon and Walcott, who have never played for England at senior level. Stewart Downing has played only once.
Berkshire boy Walcott, 17, made his first appearance for senior England team when he came on as a sub in the 61st minute.
The Arsenal starlet replaced skipper Owen, who had looked sharp but not totally comfortable, despite claiming his foot injury is fine.
Jenas broke the deadlock in the 34th minute. Stewart Downing curled a free-kick into the box and Owen headed against the bar - but the ball bounced down into the goalmouth and Jenas reacted first, stooping to nod it over the line from close range.
Owen was offside before he won the header but the flag stayed down, luckily for England.
Crouch had already been denied by the linesman when he tapped in a low ninth-minute cross from Lennon from an offside position.
Spurs winger Lennon started the game with great pace and energy on the England right.
The teenager terrorised the Belarus defence in the opening 20 minutes with his pure speed and direct approach.
After making the chance for Crouch's disallowed goal, he exploded on another jinking run into the penalty box.
He beat his man convincingly but his shot drifted over at the far post.
Belarus goalkeeper Yury Zhevnov saved from Crouch and Downing, while Campbell also went close before Jenas opened the scoring.