Velocity is one of the more important things to keep an eye on early in the season. Unlike most statistics, which can take weeks and months to really become reliable, velocity tends to stabilize pretty quickly. Big leaps forward or steps backward in terms of velocity can be meaningful even in the opening days of the year.
I went through this earlier in the week in detailing the most significant velocity changers on the Yankees. Luis Severino and Aroldis Chapman profile as the two most important players to see changes in speed, but another name to watch is Tommy Kahnle.