Spain international Mata is set to move to Old Trafford from Premier League rivals Chelsea as David Moyes attempts to freshen up his squad following a disappointing start to his tenure.
Kagawa's United career has never really taken off and the 24-year-old has been linked with a move before the transfer window closes at the end of January.
The Japan playmaker is said to be a target for his former club Borussia Dortmund and La Liga title contenders Atletico Madrid.
However, Kagawa's agent, Thomas Kroth, expects his client to remain at the Premier League club.
He told Sky Sports: "At this moment I am sure Shinji will stay. There has been no contact and so it is just rumours."
Kroth added that he does not expect the big-money arrival of Mata to affect Kagawa's future.
When asked if Mata's presence at Old Trafford could lead to Kagawa's exit, he said: "No, it is my opinion that he will stay."
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