Rashford was criticised by his manager for his display in the recent 1-0 loss at Brighton & Hove Albion - the first of a four-game run without Lukaku in the starting lineup.
Mourinho suggested that it was 'clear' why the Belgian striker had been used so often this term, as others have been unable to successfully step up when required.
Speaking ahead of Saturday's FA Cup final, however, Jenas hit out at Mourinho for his "disgusting" treatment of Rashford in particular.
"I found what Mourinho did after that Brighton game was disgusting," he told BBC Sport. "I really did. I think when you're managing a young player like Marcus Rashford as well. That was a direct shot at Rashford. It was almost like he enjoyed the failure of Rashford.
"It gave him the opportunity to go, 'well you lot shut up now. I'm right and you're wrong.' And that is not a point that you should be using on a young player."
Rashford also failed to net against Chelsea at Wembley Stadium prior to being replaced by Lukaku in the final quarter of the game.