when something goes seriously wrong for Barça, the opposite team usually does the following: press, work hard, be physical at the limit tolerated by the referee, don't give Barça players any space.
Few teams can pull it off for 90 minutes.
Atletico Madrid did it when Barça was eliminated from the CL, Espanyol did it recently when they won a game at the Copa, PSG did it in Paris, and now Roma was able to win a game that way.
It only works when Barça players are not rested enough to withstand the pressure.
Yesterday we were waiting for Roma to get tired and let Barça take over, but we had to wait until the result was 3:0.
When there were about 15 min (incl extra time) left to play, the pressure finally disappeared, Roma players got tired and there was a chance for Barça to come back. Iniesta suddenly looked good and started orchestrating the offense, along with Messi. Dembélé came finally in and added a threat. What did EV do when Iniesta started to play well? he subbed him off for Gomes.
It is easy to analyze EV's mistakes after the fact, not playing Dembélé was a big one, but his worst decision was working the key players too hard before such an important game, for not respecting Roma enough.