Despite Cristiano Ronaldo's early penalty miss in the semi final first leg at the Nou Camp, Manchester United still have a 67 % chance of reaching the Champions League final in Moscow.
However, if the Portuguese winger had given United a 1-0 lead to take back to Old Trafford, they would have been 91% favourites to win the tie.
United have drawn their last three meetings with Spanish opposition - all goalless - and a score draw in this match would see them go out of Europe on away goals.
They have only lost twice at Old Trafford in 16 meetings with Spanish teams - both coming in the knockout stages against Real Madrid in 2000 and Deportivo La Coruna in 2001.
Barcelona have won only six of their 26 visits to England in European competition but did win on their last trip - beating Liverpool 1-0 at Anfield last season (although they eventually went out on away goals).
Barca have not lost in their last seven away CL matches, and have not conceded a goal in their last four CL matches in total.