The NFL’s Standard Player Contract does not specifically prevent a player from playing basketball in his spare time, including but not limited to the offseason. However, most if not all contracts containing guaranteed payments include language voiding future guarantees if the player suffers while playing basketball an injury that prevents him from practicing or playing football.
Ben Volin of the Boston Globe has posted a portion of the standard provision regarding the voiding of guarantees. The language identifies basketball and other specific sports and activities that will trigger a voiding of the guarantees, if an injury keeps the player from performing under the terms of the football contract.
Again, the language doesn’t prevent the player from playing basketball. However, if he tears an ACL (for example) while playing basketball, the team can void any remaining guaranteed payments, both for the period he’s unable to practice and play -- and for the duration of the contract.
As noted earlier today, none of this matters if Mahomes willingly complies with a request from the team to not play basketball. But this exercise regarding offseason exercise has turned out to be a useful one, since most of the players who have accepted provisions voiding guarantees for basketball injuries may be playing offseason basketball without fully appreciating the risk they’re taking.