Keeping just seven Premier League clean sheets this season, the goalkeeper has made high-profile errors against Manchester City, Leicester City and Aston Villa, as well as Real Madrid in the Champions League, in recent weeks.
"That (Alisson's form) will have several reasons," former Reds keeper Mignolet told De Tribune.
“On the one hand, he has had a few injuries, but there is also the fact that they have been missing Virgil van Dijk for almost the whole season.
“That's a chunk of stability that's gone anyway. That was actually already a difference when I was there. Van Dijk came when I ended up on the bench.
“From the moment he started playing, we stopped goals. Liverpool with or without him is a huge difference, I think."