Van Dijk is one of the favourites for the award after helping Liverpool deliver a sustained Premier League title challenge, while Sterling has scored 15 league goals for City, their rivals for the league crown.
“I voted for Raheem Sterling,” Van Dijk told reporters. “I did what I thought and I thought he deserves it. He’s had a fantastic season. I could have voted for Bernardo Silva as well and another couple of players from Manchester City.
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“But I’m just being honest. I think he has improved a lot as a player.”
PFA rules prohibit players from voting for their teammates.
Liverpool, who have kept 17 clean sheets in the league this season, are also still in the hunt for the Champions League trophy and play Porto in the first leg of their quarterfinal later on Tuesday.
Van Dijk said he was focused on winning with the team, not on individual accolades.
“Individual trophies? I don’t know,” the Dutchman added. “Of course, you’re going to be proud, that’s something you have to be, but all the hard work in the training ground is something we all did together.
“It is to win something as a team, a collective thing as a club. Hopefully that is something that will happen.”