City agreed a fee with Parma and terms with the player but could not complete the signing because Mariga was refused a work permit on the basis of Kenya’s low world ranking.
Mariga, 22, has since signed for Inter Milan, leaving Mancini fuming.
“We couldn’t see any problems so to have the panel rule against granting him a work permit is, I believe, an incredible decision,” said the Italian.
“He is a young player and Kenya’s most important footballer and considering he has played in Sweden and Italy for the past six years and with relations between Kenya and Britain strong, I just cannot understand why he had his application refused.
“All the top teams in Italy and many others in Europe wanted him so it is very disappointing when we had agreed a deal.”
Meanwhile, the agent of Standard Liege forward Milan Jovanovic has denied a deal is in place for the Serb to join Liverpool on a free transfer in the summer.
“There are many possibilities,” said his agent Zoran Stojadinovic. “The situation will be resolved soon, but he has not signed for any club as yet.”