The Holland international is suspended for the first leg of their last-16, first-leg tie against Bayern Munich at Anfield on Tuesday.
The defensive situation is further complicated by injuries to Joe Gomez (fractured leg) and Dejan Lovren (hamstring) which means manager Jurgen Klopp faces the prospect of deploying midfielder Fabinho alongside Joel Matip against the Bundesliga champions.
Van Dijk has been such an influence following his £75million arrival in January last year that there are concerns regarding his absence, with Hyypia claiming he is irreplaceable.
Speaking to Press Association Sport, Hyypia, who will play his first game for LFC Legends against AC Milan Glorie on March 23 at Anfield, said: “I saw it right away when he came that he would be a very important part of the team.
“Straightaway he was organising the defence, he was the leader at the back, and as time has gone on he is even more important.
“He gives confidence to the players and makes many other players play better.
“It is very difficult to find a weakness. He is an ultimate centre-back and it would be a pleasure to play alongside him he is so good.
“He is so calm on the ball, he never seems in a hurry and he trusts his skills on the ball.
“It was the right decision (to sign him) because he is so important for the team, it doesn’t matter how much he cost.”
With the Holland captain missing, Hyypia said there was an expectation on other players to fill the void.
“These kinds of things give other players an opportunity to shine and take responsibility, and I hope the ones who play step up and do their job properly,” added the Finn, who made 464 Liverpool appearances and is considered one of their all-time top centre-backs.
“Of course, Alisson has been a great addition to us. He has helped Virgil to make the records we have at the moment, not conceding many goals.
“I don’t know how much he organises or talks from the back but he is a great keeper and has been so good for us.”
The draw against Bayern, unopposed winners of the Bundesliga for the last six years, gives Klopp – the last man to deny the Bavarians the title while in charge of Borussia Dortmund in 2012 – a chance to renew rivalries.
Bayern currently trail Dortmund in the league, after an indifferent season by their standards but Hyypia has warned it would be wrong to discount them.
“When the draw was made I saw everyone was saying that Bayern were not the Bayern of old but they are a very dangerous team,” said the defender, who left Anfield in 2009 to play for, and then manage Bayer Leverkusen.
“Liverpool can’t underestimate them. They need to respect them, analyse their weaknesses and then strike.
“I think one of their weaknesses is defending against the counter-attack, and Liverpool are not bad in that area. I think that can cause problems for them.”
Hyypia is taking a break from management after leaving Leverkusen for Brighton, only to resign midway through his first SkyBet Championship season in 2014.
However, he has ambitions to return.
“I will coach and manage again one day but not yet. I get some enquiries quite regularly but nothing has yet been that interesting I would jump into it,” he added.
“I wouldn’t change any decision to go so quickly into coaching. A lot of people said I made a mistake going into it so early but I learned a lot.
“You learn a lot more from the bad moments than the good moments.
“Those years when things were not going my way, I learned so much and I am much better and more ready now.”
**Liverpool Legends v AC Milan Glorie will be held at Anfield on March 23, with proceeds going to the Liverpool Foundation. For details and to purchase tickets visit here.**