It has been reported that manager Jose Mourinho's sparing use of the playmaker has caused friction with owner Roman Abramovich.
"I personally don't understand (why he left Mata out)," Grant told BBC Radio Five Live's Sportsweek programme.
"Mata was one of the best players in the midfield in the last year.
"A manager needs to take a decision and in this case it's a tough decision. Jose will know how to manage them well."