The move, approved by the executive committee of world football's governing body on Friday, is potentially controversial as the competition is not part of the international match calendar and clubs already believe they are asked to release their players too often.
However, the executive committee softened the blow by agreeing to proposals from European clubs to reduce the number of international matches and set up an insurance scheme to compensate clubs when their players are injured playing for their countries.
"The executive decided that the release of under-23 players by the clubs shall be compulsory," FIFA said in a statement.
CAS ruled in Barcelona's favour but the Catalan club then decided to release him in any case and Argentina went on to win the tournament.
"For the first time, all of these players will be insured, as of the September 1 from this current year," Blatter told reporters.
"This is a novelty, it is to the benefit of everyone, from the smallest such as Djibouti, to large federations such as Italy. Everyone stands to benefit from this."
These 18 games would be divided into so-called "double dates" each featuring two international matches in the period of five days.
The controversial February and August friendly dates, the latter having been described as "nonsense" games by ECA president Karl-Heinz Rummenigge, would be scrapped.
