Race matters in the NBA. It's the (ironically) white elephant in the room. The owners are almost all rich and white. The coaches are mostly white. NBA office staff are mostly white. And the players are mostly black. White men are typically less athletic and more "scrappy" (sorry
@scrappy2.0 ). They don't fall into the flashy, monster dunk category that dominates the NBA. They tend to either be big centers (Gasols, Mosgov, Antric, and the like), or scrappy guards and shooters (Delavedova, JJ Reddick, JJ Barea, Kirk Hinrich).
The reason that race presents itself with Delavedova is because the very facet of his game that makes him an NBA player - scrappiness - is the thing that Horford is counting against him. Horford's opinion seems to be that DV scrapiness is beneath the dinigty of the NBA, and that Horford's health should not be put at risk because some over-zealous, under-talented white guy is going to eschew Horford's ideal rendition of the game and climb up all in his grill going for every loose ball. Horford implied that there is a "brotherhood" of NBA players, and that there is an unwritten understanding that one should curb their loose ball enthusiasm rather than risk the knees of other players.
Further, the fact that DV is an under-athletic white player means that he is going to be a target of some players. K. Garnett was infamous for targeting "european players" with elbows and the like. The scrappy JJ Barea was taken out by Bynum on a play that should have resulted in a season ban for Bynum and criminal assault charges.
A lot of players fancy themselves too cool to bust ass every play and do the dirty work. It's very much a trait of white players in the NBA to be the lunchpail guys - in large part, probably, because it has to be. I think there is resentment for that, and particularly if somebody gets their panties all up in a bunch because the guy who routinely beats them to loose balls inadvertently rolled up on the leg of a guy during one scrum.