The Reds were unfortunate to be beaten 1-0 in last weekend's Premier League opener against Arsenal, with the onus now on gaining a positive result against Fulham on Sunday.
United will be backed by yet another terrific sold-out away end at Craven Cottage, with hoards of fans heading down to the capital for the 16:30 BST kick-off.
Speaking regarding his side's toughness, the boss believes a testing summer campaign has shaped his players into becoming more mentally resilient.
"Yeah, I think you can see that is a fact," said Ruben when asked if his squad are tougher than last season.