The Biancoceleste travel to Novara on Wednesday looking to make amends for their below-par performances.
Lazio looked to be heading for their 17th win of the campaign last Sunday only for the Romans to concede another late goal in a 1-1 home draw against Lecce.
That result followed their 2-1 loss at Juventus, where the Biancoceleste gave away the winner eight minutes from time.
"Against Lecce we did not get the points we deserved because we were not lucid and we lacked maturity at the end," Reja said. "I am angry because my players were distracted late in the game and that led to Lecce's goal.
"It's a pity because had we taken three points, we would have taken a good advantage in the race to finish third."
Despite having mustered just one win in their last four games, Lazio remain in third place and hold the last Champions League qualifying spot with five games remaining.
They have a three-point advantage over nearest challengers Udinese, who take on Inter Milan on Wednesday.
"I still believe that we can hold onto third spot by season's end but we must do better," Reja said.
Lazio will have to show a vast improvement if they are to take three points at the Silvio Piola stadium.
The Biancoceleste have not won away from Rome since beating Chievo 3-0 in Verona on January 29 while they will also look to halt a three-match losing streak.
Novara, meanwhile, head into the encounter on the back of a 2-0 loss at Napoli, their third defeat in their last four games.
That result left Novara second from bottom, 11 points behind safety.
The visitors welcome back defender Libor Kozac and midfielder Stefano Mauri from suspension.
However, defenders Stefan Radu and Marius Stankevicius as well as midfielders Hernandes and Christian Brocchi and the team's top scorer Miroslav Klose are all sidelined through injury.