First of all, it must be said that Alex Song has been a great signing for Barcelona so far.
He is a great passer and is strong, tough and exactly the type of aerial and physical presence in midfield that the team has needed.
But, speaking frankly, he is not the answer in defense.
Luckily for him, Barcelona's possession football will limit the amount of chances he gets to make a mistake.
There also aren't a lot of teams in La Liga that can take advantage of his weaknesses.
Unfortunately, the kinds of teams that can abuse him will only be seen in the Champions League where one or two mistakes could be all it takes to get knocked out of the tournament.
Against Sevilla, we saw the best and worst of Song.
We saw him make some great sliding tackles and interceptions that killed off Sevilla's attack, we saw him venturing forward to aid in attack, we saw his height become a valuable tool defending set pieces.
But we also saw him get lost at the back. When Sevilla came on the counter and he had to close off lanes, Song would often get caught looking around for his defender just as a key pass opened up his marker.
Sevilla's striker was fantastic as the lone man and took advantage of Song's poor positioning all night long.
The Spaniard's highlight was his goal. On that play, he simply out-muscled Song and rode him almost 10 yards before slotting the ball home.
Song is often praised for being a physical player, but Negredo showed that being a physical midfielder and being a strong, physical centre-back are two very different things.
The Cameroonian just doesn't have the right combination of brute strength and positional awareness to take on a strong striker by himself.
Song is a superb midfielder who adds lots of value, but he is not a great centre-back. His partnership with Mascherano is a disaster waiting to happen against good teams.