The Reds returned to winning ways against Bournemouth on Saturday - though must overturn a 1-0 deficit against Atletico Madrid at Anfield to reach the Champions League quarterfinals.
Van der Sar wrote for the De Telegraaf: "Last week, I spoke to Liverpool CEO Peter Moore, and together we talked about how weird it is that even with a team in such a successful period, a few defeats become a sudden concern.
“Liverpool got a first tap, then another and then something happens.
"For Liverpool, winning the Premier League remains the ultimate, but they also want to defend the Champions League. Then, after a few defeats, it suddenly becomes a difficult game against Atlético Madrid tomorrow night."