The Biancoceleste travel to the Luigi Ferraris stadium to meet Genoa tomorrow sitting third in Serie A.
They are level on 33 points with Napoli and three points behind league leaders AC Milan after 17 games.
"Napoli and Lazio have been the biggest surprise this season," Reja said.
"We, like Napoli, have a determination and a team spirit that has so far allowed us to remain in the high position of the standings.
"While it has been my best Christmas in 30 years as coach, the position in the standings only counts at the end, not in the middle of the campaign.
"But now the hardest part comes.
"We have a month with a lot of games and if we are able to overcome the next 30 days, then we can truly begin to dream.
"The Champions League would be like a Scudetto for us because there are stronger teams than us that will battle for the league title."
Genoa, meanwhile, have two new recruits in their squad as they look to avoid their third straight defeat at home.
The Grifoni have been busy in the winter transfer market with four players leaving the club, including defender Andrea Ranocchia, who joined Inter Milan earlier this week.
The arrivals of defender Luca Antonelli and striker Alberto Paloschi from Parma have strengthened Davide Ballardini's side.
Genoa head into the encounter in 11th place, 12 points behind Lazio, but still expect to give the Romans a run for their money.
"Lazio are tough opponents," Genoa midfielder Domenico Criscito said. "But we have our weapons and I'm sure we will make life difficult for them."