Cueto is set to start Tuesday against the D-backs, swapping spots with Homer Bailey, who filled in for him Sunday against the Cardinals and worked eight innings in a 4-2 sweep-completing win.
Reds manager Dusty Baker said the club is "as confident as we can be on a Sunday" but will wait to see how his ace progresses.
"It just depends how quickly his body heals," Baker said. "Some guys, it looks like they can just grow skin overnight. It depends on how your body heals and how it regenerates itself."
Cueto also missed Sunday's throwing session to allow time for his finger to heal.
"If he doesn't have a throw day, how many throws can you have arm strength-wise?" Baker said. "It doesn't take your body very long to get out of tip-top shape. We certainly don't need him to go out there and not go very long and don't need to try to go out there and compensate and maybe end up hurting something else."
Through 87 games, the Reds are one of two ballclubs to have used just five starting pitchers all year.
Cueto's injury may put that streak in jeopardy. Baker said the club has not designed a backup plan as of Sunday.
"We're sure hoping [he can pitch], because if not, we don't know what our backup plan is yet," Baker said. "We haven't even discussed one. Right now, it's just a situation where we've just got to see where he is."
Cueto leads the Reds with 10 wins this season, boasting a 10-5 record through 18 starts with 2.39 ERA.