The NFL is mulling additional changes to its redesigned kickoff. This is an unusually late development caused in part by a hectic scramble this spring to make the play safer.

An NFL spokesman said Monday that any adjustments would be "minor." But as training camps approach, there remains a rare uncertainty about the league's full plans for the play.

The league published its 2018 rule book, with updated kickoff language from its May 22 approval by NFL owners, on June 28. Any additional changes would require the NFL to take the unusual step of recalling and then republishing what was assumed to be a final document.

In a statement released to ESPN, NFL spokesman Michael Signora said: "The NFL officiating and football operations staff, with competition committee approval, may make some minor tweaks to the language to further clarify the rule. If so, that information would be circulated to all clubs and the rule book updated accordingly."

NFL data showed in March that concussions were five times more likely to occur on kickoffs than on other plays. In response, the league called together a group of special-teams coordinators in May to discuss possible changes. In a surprisingly swift action, the NFL adopted their ideas -- which essentially make the kickoff more like a punt -- for a one-year trial.

That eagerness has pushed the limits of the league's usual rule transition timetable. According to sources, some concerns arose over the weekend at the NFL's annual officiating clinic in Plano, Texas. Competition committee chairman Rich McKay was in attendance during discussions of portions of the rule that at least some officials think will be difficult to administrate consistently.

One of the concerns, according to sources, is the decision to eliminate two-man "wedge" blocks but not all double-teams. As currently written, the rule states: "After the ball is kicked, a double-team block is permissible only by players who were initially lined up in the setup zone. A double-team block is defined as two players from the setup zone coming together in an attempt to block for the runner."

That would require officials who see a double-team block during a return to know where the players were originally lined up, a mechanic that is not part of their current approach to kickoffs. It is one of several concerns that officials have about the rule. Retired referee Terry McAulay, now an NBC Sports rules analyst, told ESPN last month that the new kickoff is a massive adjustment for officials.

"It isn't getting as much press, but the kickoff change, this may be the biggest change I've ever seen," McAulay said. "People know so little about it. There are so many restrictions on what either side of the ball can do. ... I spoke with a special-teams coordinator who is excited about it, and I know we all feel it can be great for the game. But [officials] have to wait and see. There are a lot of intricate rules in terms of what [players] can and cannot do."

The timing of the next round of changes is unclear. Referees and other game officials will circulate through training camps this summer to better understand how teams plan to implement the new rule.