Messi is the best we have ever seen but he's not as otherworldly as he once was. His performances in the knockout stages over the last 2 or 3 seasons have been less than they were in his true prime. You look at last season and he was stifled by Roma in the 2nd leg. Often if their physically out-matched in the centre of the park, Barca can struggle to supply him. Roma, Juve and Atletico showed that in the last three CL campaigns. If we do meet, we would offer the same issue ourselves.
Utd for however much of a mess they are at the back could potentially cause them issues with the likes of Pogba, Matic and Herrera. Then you need the speed up top to take advantage of the likes of Roberto, Pique and Alba. They do have that in Martial and Rashford.
I'm not a calling a Utd win at all. Just think there is a potential upset if the luck works out for them at the back.
This is probably Messi's best season in about 5 years, certainly numerically (he is currently at a goal or assist every 51.7mins in the league, 43.4 mins in the CL; whereas the last 5 years it was 65.2/61.6/65/55.3/66 in league and 97.9/62.3/90/76.4/78.8 in CL - you need to go back to 2012/13 where he bagged a goal every 45.6 mins in league but 82.3 in CL to better it)
This is also Utds worse defense in about the same time - currently they have conceeded 43 goals and had only 7 clean sheets - the last 5 seasons for them are 28/29/35/37/43 and 18/14/15/10/12 - yes, that means they are as BAD defensively as under Moyes in terrms of already conceeded the same number of goals as the whole season with 6 games left, and 5 clean sheets fewer.
There is about as much a chance of an upset as if it were Everton as long as Messi and Suarez are fit