Maybe he had a knock. More likely though, it was because he wasn't producing anything, whether early in matches (when fatigue couldn't be the issue), or afterwards. Aside from the goal against Switzerland (which was off a deflection), he had a pretty miserable World Cup. At least bringing on Olsson gave Sweden a bit more pace in attack. Anyway, Forsberg is just one player. Your argument was that the whole team was exhausted. I saw little evidence of that. .
Ok. So Forsberg had the energy to carry on, in your opinion, and was subbed out because Olsson injected some pace to Sweden. But then why was Olsson subbed on for Forsberg rather than another Swede? Sweden needed goals and a player like Forsberg was an important piece of Sweden's attack. Sweden also dropped their concentration and lost possession. Their sloppiness was due to fatigue, not because they're bad with the ball. In matches against Germany, Italy, France and Netherlands, they gave their opponents a hard time. England are a good side, but they dispatched Sweden with ease because Sweden was fatigued. They had some players who could play with loads of energy, but others were short on energy. Players either exhausted themselves, like Forsberg clearly did, or they reserved the manner of their play, like Berg did, in order to see out the game. All I'm saying is on a different day, England would have played against a tougher Swedish side. That's because on this occasion the side they beat was weakened due to fatigue.