That's UNITED's psychology not the other team. You can't predict how someone else will react. Psychology isn't as cut and dry as you make out, for instance exams: You can say that these put immense pressure on students, that's a given. How can you predict what result that has? Some thrive off it and ace the exam, some will collapse into a panic attack and fail the exam. Apply that to my football example, and even to your United one. Everyone knows how good Barca are, to United that may be a challenge to thrive off, to another team it's a reason to give in before the game even starts. Play your game, there's no point us hammering a team to send a message, because we have no idea how that message will be interpreted. Sure, if we won by a huge score, a random fixture with Germany could cause them to panic into collapse, it could also force them to play out of their skin against us. THAT is real psychology.
Look at the media, if we were 3-0 up versus Bulgaria, we push for a fourth and they hit us back 3-1. So we push further and concede a 2nd on the break, and then in the dying minutes a sloppy set piece and they equalise. It's not likely to happen, but it could, and the media is far more likely to focus on England collapsing than Bulgaria performing. And they won't say we were too over zealous and not cautious enough, it'll be "players like passion", "they don't care for the shirt", because that's just how they roll.
And you seriously overestimate San Marino. They're semi-pro at best, they're calling on butchers, bankers, dentists etc. to go out and play against one of the most fearsome attacks in international football. They're international by definition only, there's a reason they've lost every qualifier they've played.