Hernandez started the Championship match for Leeds against Villa at Elland Road on Friday evening and was substituted in the 42nd minute.
The Spanish attacking midfielder was playing well until he got injured, and Villa stepped up their game in the second half.
The Whites took the lead through Sweden international defender Pontus Jansson in the 19th minute, but Henri Lansbury scored the equaliser for the Villans on 71 minutes.
Leeds legend Gray was following the match, and the 69-year-old former Scotland international winger believes that the injury to Hernandez changed the game.
Hernandez was looking very creative against Villa, and his absence was felt by the West Yorkshire outfit in the second period.
Even then, a point against playoff-chasing Villa is not a bad result, and Leeds should be pleased with the final result.