I don't think we are going to sell him, i dont want us to sell him and i love him. However, if we get 25 million for a defender, i think we'll survive. We can bring in a good prospect defender like coates for less than 10 and spend 15 on a winger or whatever. We have 4 good cb's right now, going down to 3 won't kill us if we improve the first team overall, especially since I expect Agger to play 25 games in the league tops.
The issue isn't whether he can be replaced, because he can... players by their very nature are finite - so eventually you have to replace them. But that doesn't always mean we should willingly force that issue, as it creates a headache.
What I have issues with when people talk about selling some of better players, arguing that with the proceeds we can invest in a number of positions, is that they are not taking into account the life cycle of the squad, ambitions of the club short, medium and long term.
What i mean by this can be highlighted by a few observations of current issues the club are experiencing and previous ones.
Last season we struggled to score goals, if we compromise the defence by selling Agger or Skrtel we could quite conceivably start to concede more. That's a headache i'm sure we're rather not have, considering we already have an almighty one in our forward positions.
I also recall a few years back, people argued vehemently against buying quantity over quality, because it diluted the overall ability of the squad and in games where that extra bit of ability was required we went missing (last piece of the jigsaw statements) - Which begs the question, how does selling a quality player with the argument that re-investing that money in a number of players put us in a better position? There is no guarantee that it will.
Life Cycle - at the ages both Agger and Skrtel are, they probably have around 5 seasons of real top quality football before they start a decline in their performances, age catches us all up. If we replace Agger with say Coates, it may be 3 or 4 seasons before he reaches a level that we would recognise with Daniel. By that time it's probable that players like Lucas, Suarez - certainly Gerrard, potential Reina, Johnson etc will all have left. So it offers little continuity.
Building a winning team is not only about have good quality players, but players reaching an optimum level of consistent performance at a similar time to their team mates. Just look at the West Ham team that later got relegated around a decade ago, it boasted lots of house hold names that have gone on to become successful. The club however wasn't.
My opinion, for what it's worth on our best players is simply - do everything we can in our power to keep them. If our owners really want to become more competitive, whilst prudent trimming of fat in our budgets is absolutely fine and should be endorsed, recycling a players value back into the squad because they're not prepared to invest the money required to build a successful team, is not.