Murphy has claimed that making in-form players unhappy can become "a negative influence on the team".
He told BBC Sport: "Manchester United won 4-0 against QPR, they played really well and Juan Mata scored. What more can he do?
"The problem for Man Utd this season will be leaving big names out. Without Champions League football, will Louis van Gaal be able to keep those players happy?
"Because it does not matter how much money you earn, if you are not playing you are unhappy. And those players could become a negative influence on the team."
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