The hosts needed a late Stefano Mauri goal to secure a point against Verona at the Stadio Olimipico as Reja's men were dealt a blow in their UEFA Europa League quest.
They now sit four points behind Inter in the final guaranteed European spot with two games to go, having led twice on Monday in a game that saw both sides finish with 10 men.
However, due to Coppa Italia finalists Napoli and Fiorentina both featuring in the top five, a sixth-placed finish is likely to be enough for a Europa League berth and head coach Reja has not given up on the possibility.
"We still believe in Europe," said Reja, whose side face Inter on Saturday.
"It often happens that we go in front and can't maintain a lead. Perhaps it's a drop in concentration levels, we always make a few mistakes.
"The draw is no good to either team. A side like ours can’t be scoring three goals and conceding just as many. We should've made Verona fight harder for their goals."
Reja took over from Vladimir Petkovic in January, returning to the club for a second spell in charge.
The 68-year-old said he would not use the off-field situation that surrounded Petkovic's drawn-out departure as an excuse and acknowledged that he did not yet know how to make his side more consistent.
"When I arrived I told the lads that I didn't come here for a transitional season, but to have an objective," he added. "We were getting close, but never made that definitive step up in quality.
"I don't know if the limitation is with the coach or the team. I don't know. I can't say the squad didn't give it their best."
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