In actual fact the game could have had many outcomes:
Scenario 1 - Lehman not sent off, Arsenal 1 nil down, Barca grew in
confidence and could have add to their tally
Scenario 2 - Lehman not sent off, Arsenal 1 nil down, could have
come back to win the game
Scenario 3 - Lehman sent off, Arsenal won by 1 or 2 goals
Scenario 4 - Lehman sent off, Arsenal 1 nil up, but end up losing
2-1
We cannot say for sure with 11 men Arsenal would have beaten Barca,
yes with 11 men Arsenal would have stood more of a chance, but to
say that with 11 men Arsenal for sure would have won the CL would
be naive to say the least.
Because scenario 1 could have happened and even with 10 men
scenario 3 could have happened too.
Nothing is certain in the world of football, because so many facts
would change the outcome of the result. If you consider "kelong"
theories, then more possibilities could have happen too. 
As a case of today, just a sending off and not awarding the goal to
Barca in the first half could have probably lead to 4
scenarios.
