Horford looks old. Hard to be an effective undersized center on defense and when rebounding when you start to lose your quickness / explosiveness advantage.
Exactly what I said before we signed him, when everybody insisted that his game would age well because it is 'skill based'.
People don't get it. When a really big dude (like a Ewing, or a Shaq, or a Robinson / Duncan / Garnett) loses their athleticism, they can remain effective because they still have the size and length to shoot over guys.
What was KG's favourite play in the playoffs once he started getting older? Rondo would lob straight over the defence, and KG would just go up and get it, and put it in. Length - it's very important when you lose the ability to run and jump.
When you lose the ability to run and jump, and you are also at a size disadvantage most nights, then how are you supposed do anything? Skill will only get you so far. You can make a nice pass or two, hit some open jumpers if the defence gives it to you, but that's it.
Lopez is nowhere nearly as as skilled as Horford - not even in the same stratosphere. But even despite Horford's nice stats in game one, the eye test suggested that Lopez had more impact then he did in both games. As the old saying goes, you can't teach size.
If Horford was a strong, angry, physical brute - then maybe his passion and energy could overcome those physical limitations somewhat. But hes too nice, too polite, too placid. That's why he's going to age rapidly, decline rapidly, and quickly become one of the worst contracts in the NBA.
To make matters worse, Horford was actually never really a GOOD rebounder. There were a couple of years where he was an above average one, but most of his career he's only been average on the boards. So it's not like he ever had an amazing nose for the ball, perfect timing, the ultimate in box out skills, etc. He's just not a naturally great rebounder. Take away his athleticism and that will drop off exponentially.