Steven Gerrard was among four high-profile absentees from training this morning as the debate over his availability for England's crucial Euro 2008 double-header with Israel and Russia continues.
The Liverpool captain has played only once since breaking a toe against Toulouse three weeks ago and Reds boss Rafael Benitez has expressed doubt as to whether Gerrard will be fit for both games, when England will require a minimum of four points to retain any realistic hope of reaching the European Championship finals in Austria and Switzerland next summer.
For his part, Gerrard has assured McClaren he will play.
However, the England coach decided to let Gerrard continue his rehabilitation in the gym as the rest of his squad were put through their paces for the first time at Arsenal's London Colney training complex.
Gerrard was not alone in missing the main session.
Captain John Terry was also absent after reporting for duty last night feeling slightly under the weather.
Manchester United's Owen Hargreaves was also restricted to gym work as he continues to battle against a tendinitis problem in his knee which has affected the start of his Red Devils career.
McClaren is confident both Terry and Hargreaves will train tomorrow, which will be a welcome relief considering the uncertainty over Gerrard and the news that Frank Lampard has effectively been ruled out of Saturday's encounter with Israel at Wembley.
Lampard was always a major doubt after sustaining a thigh muscle tear in training with Chelsea on Friday.
The midfielder did link up with the England squad as planned last night but after a quick assessment by the FA's medical staff, the 29-year-old was sent back to Chelsea.
He will report once more on Saturday for a reassessment of the injury but it seems highly unlikely Lampard will be fit in time to face Russia in the second of England's vital next two games on September 12.
McClaren did have everyone else available though as preparation for the Israel game began.
For the first time in three years, Emile Heskey is part of an England squad with a realistic chance of playing after his shock call-up on Sunday.
Everton's Joleon Lescott was also put through his paces after being introduced to the senior squad for the first time following the withdrawal of Portsmouth's Sol Campbell.