The Tottenham winger missed training on Friday after reporting a sore back and after further tests on Saturday morning, it was decided to release him back to his club for further treatment.
Lennon, who has not played for England since the World Cup, is the second player to leave camp in as many days after Phil Jagielka's withdrawal with a hamstring injury.
Downing won the last of his 23 caps against Slovakia in March 2009, since when his career was put on hold by a foot injury that ruled him out for six months.
The Aston Villa player did make his comeback in time to try to force his way into Fabio Capello's World Cup plans but, after making the squad for England's friendly win over Egypt in March this year, did not even make it into the Italian's 30-man provisional party.
It means Downing will link up with former Middlesbrough team-mate Adam Johnson at international level for the first time, although it is the younger of the pair, now at Manchester City, who is far more likely to start as England look to preserve their 100% record against the surprise Group G leaders.