I really don't get this.... they have played (ignoring pre-season) 6 competitive matches of a basic 90 minutes (assuming they played all the time in every match) between 10th and 31st August. These are supposed to be professional, fit young men at the peak of their careers.
Compare this to, say, professional tennis players, who during the tournament circuit, will play up to 4 hours, in full heat, with very short breaks between sets. If they win the match, they will be back on court 2 days later doing the same again (and sometimes fitting doubles matches in between singles) - a far harder and more demanding schedule for their bodies to cope with, but they do it almost year round, and major injuries are relatively rare compared to football.
I get that body movement is different for the two sports, although if football requires more running ability and stamina, tennis is certainly more demanding in terms of short sharp movements and twists and turns and stretches.
Perhaps, as I've said before, the answer lies in the training, fitness and preparation rather than the number of matches.