Despite Birmingham's City League Cup Final win, they were relegated to The Championship on the final day of the 2010–11 Premier League Season.
On 1 March 2009, Alex Ferguson named Foster in goal for the 2009 League Cup Final at Wembley Stadium.
After keeping a clean sheet in normal time, Foster then made a save from Tottenham's Jamie O'Hara in the consequent penalty shoot-out.
However, he had been attempting to play with a ruptured ligament in his right thumb, which became aggravated, necessitating surgery which ruled him out of United's title run-in, the Champions League Final against Barcelona and England's 2010 World Cup qualifiers in June.
Following speculation that United were in the market for a new goalkeeper to replace the soon-to-retire Edwin van der Sar, Foster signed a new four-year contract with the club.
However, he never made an appearance for Stoke, and spent time on loan with Bristol City, Tiverton Town, Stafford Rangers, Kidderminster Harriers and Wrexham.
He switched permanently to Manchester United in July 2005, but again struggled to break into the first team, and he spent two successive seasons on loan to Watford from August 2005 until the end of the 2006–07 season.
Foster played in 37 of their 38 league games, only missing the final game of the season with a minor groin strain.
He kept 10 league clean sheets, equalling West Brom's Premier League clean sheets record, as the club finished in 10th place, their highest league finish in 30 years.
Following Birmingham's relegation to the Football League Championship at the end of the season, Foster was loaned to West Bromwich Albion, and moved there permanently in June 2012.