Lorik Cana's fifth-minute goal was enough for Lazio at the Stadio Artemio Franchi, helping them move up to eighth in the Serie A.
Reja said it was a key win for his side after their shock UEFA Europa League exit at the hands of Ludogorets on Thursday.
"We did not deserve to be eliminated in the Europa League, even if we shot ourselves in the foot to a certain degree, so this victory was needed to boost confidence," Reja said.
"We had several chances to finish off the game, including hitting the woodwork, but we must learn to be more ruthless. We can't have so many opportunities and still suffer to the final whistle."
Striker Brayan Perea, 21, is set for a rest and Reja said he had confidence in his youngsters to keep earning his team points.
"The young players are talented and we have to believe in them when seeing them train well during the week. We are also looking to the future, but that means giving them experience," Reja said.
"Brayan Perea doesn't hold up the ball as well as Miro Klose does, but tonight and against Ludogorets he worked very hard indeed and now he needs to rest."
