Courtney Lawes gave England manager Martin Johnson a World Cup scare after injuring his neck in a behind-closed-doors trial match.
The Northampton lock was treated on the field by a team of medics for 10 minutes before being carried off on a stretcher with his neck in a brace.
Lawes, 22, was later transferred to Charing Cross Hospital in west London by ambulance, where he was due to undergo precautionary scans on Thursday night. The earliest indications from the England management were optimistic.
Lawes missed the whole of England's victorious Six Nations campaign with a knee injury and it would be a significant blow to Johnson's plans if he was to miss the World Cup too.
Saracens wing David Strettle was also carried off, having suffered an apparent leg injury in the first period of a full-contact match at The Stoop, played in two 20-minute halves.
Johnson mixed the teams up - there was no distinct A and B team - but there was one notable absentee in Mike Tindall, who is getting married on Saturday.
Leicester number eight Thomas Waldrom is carrying a calf problem and also played no part but Manu Tuilagi showed glimpses of his growing promise at outside centre.
Jonny Wilkinson also made an impact after coming on, helping create a try for Mark Cueto early in the second period as the team in white won 11-10. Tuilagi and Tom Croft scored for the black team.
Johnson is understood to be cutting six players from his 45-man squad this weekend ahead of England's first World Cup warm-up match, against Wales at Twickenham on August 6.
England then play Wales away before Johnson has to finalise his 30-man World Cup squad on August 22.