Linotype has a nice feature article about football jersey typefaces:

http://www.linotype.com/2710/footballjerseyfonts.html

However, they display a Paraguay jersey that uses the exact same typeface and the closest match they can suggest is "Russell Square," which is similar, but only vaguely. I tried a "What the Font" search on myfonts.com and searched by letter characteristics on identifont.com, but came up empty.

The shirt you reference is from the 2007-2008 season for AEK Athens, and appears to have been created by Puma. The typeface may be custom to the company, and not commercially available.