Lionel Messi appeared to suffer a leg injury of some kind after his goal in the 38th minute against Paris Saint-Germain. Despite rubbing repeatedly on the back of his leg, Messi finished the first half but was unable to continue when the second half began in Paris.
Cesc Fabregas has replaced Messi and while it may not cause Barcelona too much consternation today with an away goal already secured, there will be definite concerns for the health of their Argentinian star moving forward. Several Barca players have been fighting with hamstring issues over the past months and the last thing Tito Vilanova will want is another on-going injury problem with one of his key players.
For the moment, we don't know the extent of the injury but clearly it was either bad enough to necessitate the substitution or, as is more likely the case, Barcelona made the right decision and did not want to risk further aggravation of the injury by keeping Messi in the match.