Garnacho broke down in March, with it predicted his season was over. However, he could now start in today's FA Cup final against Manchester City.
The Athletic says staff were surprised as he 'worked as hard as possible to return'.
Garnacho would go into Carrington on his days off, while also completing 'monotonous hill walks at the back of the training ground'.
The winger threw himself into his gym work, something that 'pleasantly surprised coaches'.
Garnacho also accepted United's decision to block his call for Argentina's U20 World Cup without fuss.