"We have talked about different scenarios," Gibson said. "We'll see how we get there."
After giving up two runs in six innings of a no-decision Tuesday, Anderson is 6-4 with a 3.58 ERA in 11 starts since his early-May promotion from Double-A Mobile. Including his time at Mobile, Anderson has logged 99 1-3 innings, nine short of his career high set in 2010, his first full minor league season. The D-backs have a floating ceiling on Anderson, Gibson said, a lot depending on the shape and success of his outings moving forward.
"You can sit here and try to predict it, but what is going on is going to predicate what happens. We'll continue to observe him. If we skip him, we may skip.
"Either that or we could just add a sixth starter."
The most likely sixth starter appears to be Bradley, who has pitched well at Mobile after missing about five weeks with right elbow soreness. He is 1-0 with a 1.78 ERA in five starts at Mobile while gradually returning to the form that made him the No. 9 ranked prospect in Baseball America before the season and No. 12 at midseason.
"What I want Archie to do is continue to throw the ball well like he did his last start, get on a roll, and be prepared for an opportunity should it arise," Gibson said.
Bradley had his best start at any level this season Saturday, when he gave up three hits and two runs and struck out 10 against Charlotte. Bradley's fastball was clocked in the 91-95 mph range, and he has been working on adding a sinking fastball, Gibson said.
"He's pitching more," Gibson said, a nod to Bradley's use of his secondary stuff. "He's throwing all of his pitches, his breaking ball, his changeup as well. That's kind of what you want to see. You can't just rare back and throw it here. He's going to settle in a little bit from where he first started. He's working on a two-seamer as well."
It has been an eventful year for Bradley, as it has for all the D-backs' pitchers, who began spring training a week early because of their season-opening series in Australia on March 22-23. He was in the running for a spot in the rotation, and his agent was publicly upset that Bradley was not promoted when a rotation spot opened in April. Bradley was shut down with elbow soreness shortly thereafter, after making four starts at Triple-A Reno.
"I think he probably threw more coming into spring training than he usually does . . . maybe overextended himself a little bit before we put him on the disabled list," Gibson said. "I think it has kind of helped him understand what he has to do. He's good. He's strong. His mechanics are good. He is really working on his total pitch package and is becoming more comfortable in utilizing it."
The D-backs promoted Adam Eaton in September 2012 and Chris Owings last September in an attempt to get the natural butterflies out of the way before a full immersion the following springs.
"When you first come up, it is quite different," GIbson said. "There are some people who are exceptional. They take off and never seem to look back. Any experience he could get would certainly be valuable going into next year."
Cahill gave up eight hits and seven runs, three earned, in four innings in Wednesday's 11-5 loss to Detroit on Wednesday, and it seems likely that the D-backs will keep sending him out, at least in the near term.
Nuno, obtained for Brandon McCarthy, has made three starts for the D-backs, and after a strong first outing, he has given up seven runs and three homers in 9 2-3 innings. Collmenter is certain to have a role on the 2015 staff -- either in the rotation or as a valuable piece of the bullpen.