I assume you mean the season before last and what spin? You can use rubbish statistics to prove most things - for example I could have said Alisson only conceded 20 goals last season so is clearly far better than De Gea's ever been but there's no context to that though. Expected goals stats give you proper context and allow you to judge the performance of keepers (and attackers) regardless of the quality of the side they're in or even league they're playing in.
Again, based on the quality of chances conceded Liverpool would have conceded 35.75% more goals than they did with an average (literally speaking) keeper in goal. Utd should have conceded 6.7% fewer goals. That's a 42.45% swing. You asked how well Alisson would have done playing behind Jones and Smalling and the answer is considerably better than did De Gea last season. In fact only Wolves, Leicester, Brighton, Southampton, Fulham and Huddersfield's keepers would have done worse assuming they performed at the exact same level as they did for their own sides.
You can justifiably say Utd's defence is sub par but last season so was De Gea.
