The 22-year-old had been set to join the Premier League club but British work permit problems halted the transfer and the Italian champions quickly seized their chance on deadline day.
"Inter have bought McDonald Mariga in a co-ownership deal. Mariga has signed a contract until June 30, 2014 and will wear number 17," an Inter statement said.
Jose Mourinho's Inter, who earlier loaned struggling winger Mancini to AC Milan, had been looking for a new midfielder after selling Patrick Vieira to Manchester City.
The Serie A leaders last week pulled out of a deal to take Valencia's Manuel Fernandes on loan and had also been chasing Palermo midfielder Simplicio and Lazio's Cristian Ledesma.
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