The Arsenal legend reckons the Liverpool centre-backs is often 'stood like a statue', which has resulted in the Reds conceding.
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Back-to-back defeats for Jurgen Klopp's men against Nottingham Forest and Leeds in the Premier League have left them in ninth spot.
And the Merseyside outfit could slip further down the table should they fail to get a result away at Tottenham on Sunday.
And Gunners hero Keown, who won three top-flight titles at Arsenal, has offered advice for Van Dijk at how he can get back to his best.
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Keown has pinpointed three occasions against Manchester United, Crystal Palace and Leeds when Van Dijk could have stopped Liverpool's opponents from scoring.
Speaking on White and Jordan, he explained: "I though Van Dijk was probably one of the greatest centre-halves we've ever seen.
"And I think that's in question right now. I think he could be the all-time great.
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"But I do feel that what he's doing is, at times, instead of actually going and pressing the ball, smothering his opponent, he's stood like a statue.
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"He tries to make himself look big and he's hoping that the ball is going to hit him, and I just feel that the instinct needs to be to go towards the ball.
"Do you remember the game earlier this season where [Jadon] Sancho scored that very good goal and you saw [James] Milner was in his ear wasn't he? Milner was bang on it.
"And the one before against [Wilfried] Zaha, Crystal Palace, it's like he's running in an Olympic lane. Get across! Does he feel like he doesn't want to get embarrassed? Someone skipping inside of him?
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"Why doesn't he want to risk going in? I think you need to do that, I think you need to do some of the rough and tough stuff as well.
"I thought it had gone away when he played [Erling] Haaland, he and [Joe] Gomez were quite magnificent."
Keown then stated how he should have done better to stop Crysencio Summerville's winner for Leeds when Patrick Bamford's lofted flick fell to the Dutch youngster.
He added: "But against Leeds, you can't sit off and hope, you've got to go in there and stop the ball from rising and it hits the back of the net.
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"It's do or die in that situation, and he doesn't seem to have that instinct to go.
"When he gets that [instinct to press], I think he'll probably be the best defender we've seen, all around.
"His passing is magnificent, he's a giant in the air. He's ahead of everybody on a lot in most categories, but he isn't when it comes to blocks."