Disagree. It was "probably not," battery (see below) and it is inconsequential if it occurred off the ball. In the US, the courts that have looked at "assault and battery" on the field of play use a different standard that focuses on the rules of the sports and "consent." Both criminal and civil assault and battery require the touching be either unlawful or without consent (which forms a complete a defense).
In the context of sports (youth, adult-amateur, professional), the basic rule is that a player would need to not only violate the rules of the game, but commit an act that is so far beyond the rule makers' and the game participants'
contemplation that it was not reasonably foreseeable in order to negate presumed consent to be subject to criminal prosecution. (see,
Sports Law Blog)
In this case, you have a player tackling/pushing another player from behind, clearly not playing the ball, and "using excessive force." The rules (aka Laws) of soccer contemplate this type of action (Law 12) and require the player to be sent off (if a referee sees it). This type of "excessive force" behavior happens all the time in soccer at various levels (trips from behind, dangerous tackles, elbows to the ribs, etc.).
The only way I see this as potentially rising to the level of exceeding the express and implied consent within the sport of soccer would be if the player used a weapon or foreign object of some sort OR once the player was on the ground, kicked or began punching the defenseless player. Once the physical contact exceeds the reasonably foreseeable "fouls" then consent is exceeded and criminal charges may result.
See,
State of Washington v. Shelley (1997) for a discussion of how consent in sports is a defense to criminal prosecution. California and Washington follow similar paths because they are both in the 9th Circuit.