Sure, all you need is something heavy if you want to increase strength at the very end of it. However, it's still much easier to go to a gym where there's equipment where you can easily lift progressively more weight as strength increases, instead of trying to find a bigger stone in a field.
Also, crossfit is not just about core strength, it's main thing is that it can increase peak strength and endurance strength to elite levels. Reality does not support that, Crossfit is literally just expensive circuit training, with a much higher risk of rhabdo and/or injury. No way would I want a multi-million dollar investment in a player go to waste because he did a stupid training program that led to injury.