Michael Ballack has been added to the national team only for the recent draw with Croatia, which was only his second international game. But recently he was really impressing in the Bundesliga with Bayer Leverkusen and I think he will be part of Erich Ribbeck's side at Euro 2000.
The thing about Matthäus is that:
Ribbeck plays with 3-5-2, this means in defense we have two centerbacks and one Libero. Quality players like Christian Wörns, Markus Babbel or Thomas Linke are centerbacks. But there are few players in Germany that could effectively play the Libero position. One is Jens Jeremies (Matthäus' successor on the Libero position at FC Bayern), but he is more effective in the defensive midfield. The other one is Leverkusen's Jens Nowotny, but he is better on the centerback position.
So there are few alternatives - and Lothar showed brilliant football in his last year for FC Bayern München, so Ribbeck's decision to take Lothar Matthäus (39!!) to Euro 2000 was a no-brainer.
As I mentioned before, Christian Wörns is no possibility for the Matthäus position, but he isn't even in the first team as centerback! This has two reasons: At first he played awfully for Germany at the Confederations Cup last July. He was one of the weakest players for a very weak German national team (0-3 vs. USA!!). Everyone saw, that he completely lacked of motivation to play for Germany.
The second thing is the recent slump of Borussia Dortmund. They didn't won a single game out of the last 11, they even lost the last 6 games. Dortmund, the team with (I suppose) the highest or the second-highest payroll in the league, with stars like Ikpeba, Wörns, Möller, Lehmann, Bobic or Evanilson are now just one point away from the relegation zone!