The Brazilian netted a last-minute winner at the Friuli, much to the delight of the Biancocelesti boss, who hopes to strengthen this month
The Biancocelesti appeared doomed to defeat when Ogenyi Onazi was dismissed shortly after half-time, with the vistors already trailing to Antonio Di Natale's early penalty.
However, Lazio twice drew level with 10 men, through Antonio Candreva and then an Andrea Lazzarri goal, before Hernanes, who has been criticised for his below-par performances this season, netted a last-gasp winner.
"Before the game, I told the guys that they had not win away from home for too long and that today was the perfect opportunity to end this negative statistic," Reja told Sky Sport Italia.
"When I saw Udinese go into the lead for a second time, however, I thought that the team was not going to do it. Instead, the guys put in a great effort and then Hernanes found that injury-time goal.
"I know him and I know that he is an extraordinary player. If he has that kind of space, he is one that always punishes the opponent.
"Then, before the game, after my team-mate, he clenched his fists and rallied his team-mates.
"He wants to be an important player for this team and one that can be very useful for Lazio."
Hernanes, of course, has been linked with a move away from Lazio but Reja, who returned to the club earlier this month to succeed Vladimir Petkovic at the helm, revealed that he is solely concentrating on strengthening his squad before the January transfer window closes.
"I've spoken with the president [Claudio Lotito]," he stated. "We are in agreement on the things we need to do.
"We have Juventus and then Napoli [coming up in Serie A], very challenging games, so I would like to have some more options."
Lazio's victory in Udine lifted them to seventh in the standings ahead of Sunday afternoon's games.