I can't agree with that. Alisson is such an upgrade on Karius, it's insane. Since VVD came in, we had the best goals conceded rate since January. With Alisson coming in, our solidity at the back should be even greater. I expect us to concede less than 30 league goals and in fact, you're free to quote me later on, we'll concede fewer than Mourinho's lot. Here are some decent stats on what we have brought:
Expected goals (xG) measures the likelihood of a shot ending in a goal based on several variables such as assist type, shot angle and distance from goal.
Based on quality of shots he faced, Alisson should have conceded 8.31 more goals than he did.
He was the leading goalkeeper in the Serie A Expected Goals on Target (xGOT) standings, which shows how many goals a team should have conceded compared to their actual total.
The only Premier League players to fare better than Alisson last season were 
Manchester United's 
David De Gea (+13.66) and 
Nick Pope (+10.26) of 
Burnley.
xG prevented
Alisson      +8.3
Karius       -0.6
Mignolet    -0.8
Save %
Alisson      79.3
Karius       68.9
Mignolet    57.3
Save % shots in box
Alisson      72.8
Karius       61.8
Mignolet    47.6
Save % shots out box
Alisson      93.0
Karius       90.9
Mignolet    88.2
Accurate sweepings
Alisson      41
Karius       13
Mignolet    9
Clearances (inc. punches)
Alisson      39 
Karius       15 
Mignolet    13
Alisson operates in a totally different planet to Karius.