Then you don't know the rules properly

. When 2 players of the same team collide and go down to the floor with potential injury (it doesn't necessarily needs to be a head collision) the referee has to stop the play. And he should let them continue the match if they can without making them wait in the sideline.
Look for it in the latest rules advices from Fifa and UEFA. Even journalists in the Spanish TV were talking about it during the match and in fact Barcelona sent a report of it to UEFA, as many other clubs do when a referee makes technical mistakes during a match, such as whistling offside from a throw-in.
But you can't provide the rules either? Easy argument to make.
The only exception I can see to the rules is the line:
"players from the same team have collided and need immediate attention"
This is worded the same was as the head injury exception, so for me is covering the fact that the main rule is focussed on collisions of players on opposite teams. And in the case of the two Barca players (who let us remember were desperate to come back and play no more than a minute later) they clearly didn't need 'immediate attention'. Imagine if this rule were enforced all the time, if one team was breaking 3 on 2, two defenders could run into each other, 'injure' themselves and the ref would be forced to stop play.
I keep up with the rules and would love to see the one that says players can be allowed immediately back on if they're both on the same team. That seems massively counter-intuitive to me. The only exception I know is that you have to have a keeper on the field. So if the goalkeeper is injured, you can't start play til he or a new goalkeeper is ready to go.
Just because Spanish newspapers say something, doesn't mean it is right. And I'm not trying to be a d*ck, I just genuinely can't believe there's a rule that says if two players from one team are injured, you let them right back on, but for all other situations you don't.
Aha, and after much digging I've found it. In the 2012 'advice to referees' released by FIFA, there is indeed a clause:
Advice, 5.9: INJURIES
Players who are injured are required to leave the field under either of two conditions: The referee has stopped play due solely to the occurrence of a serious injury or the referee signals approval for anyone (team official, medical personnel, etc.) to enter the field to attend to an injury (regardless of whether that person enters to assist or not and regardless of why play was stopped). Goalkeepers are exempt from this requirement, along with any field player who may also have been injured as a result of a collision with the goalkeeper, as well as teammates who collide. The failure of a player to leave the field when required to do so may be considered cautionable behavior.
Fair enough, didn't know about that.