My sources explained that Favre suffered depressor labii inferioris and depressor anguli oris muscle strains with the possible involvement of the mentalis muscles. The condition is termed 'Poutman's Syndrome' that is most commonly suffered by extremely disgruntled persons, usually children. It's symptoms include an extended and quivering lower lip.
This condition will not effect his ability to throw or run, but may inhibit Favre's ability to properly enunciate audibles at the line of scrimmage.
Example of Poutman's Syndrome sufferer's symptoms: