He's been with us five seasons and has 6 reds in all comps. This problem is overstated.
I would argue that his play style of riding that line brings indiscipline in the opponents as well, results in bookings for them at times an intemperate play which is not to their benefit. The edge that Romero has is important. We need players to be more aggressive on balance.
And for a player who has a reputation for being booked every week, he's only got 4 reds in the league for Spurs, despite being a marked man according to most. One could read that as him knowing how to play on a yellow, which is critical for a first choice CB.
Our best defenders in my lifetime supporting Spurs have been about positioning, organizing and leading by example - the aggression thing is bollox IMHO