Rockies manager Jim Tracy said the extra start will allow Cook, 32, the Rockies' career leader in wins (69) and innings (1,215 1/3), to increase his stamina. Cook threw 99 pitches in 7 2/3 innings at Colorado Springs on Saturday and gave up eight hits and six earned runs. Relying on his sinker, he forced 13 ground-ball outs.
Colorado Springs pitching coach Doug Linton and organization pitching instructor Bo McLaughlin monitored the start for the Rockies.
"It's not so much how he is going to pitch, how many ground balls is he going to give up," Tracy said. "It's the stamina part of it -- building additional stamina and feeling like you've crossed every bridge before putting him back on the Major League mound."
Cook, who hadn't even faced hitters before suffering the injury during the spring and made three rehab starts at Double-A Tulsa before making the start for Colorado Springs, said he has been throwing four pitches effectively. Although the Rockies felt there was reason to delay his return, Cook said he is headed in the right direction.
Last year, Cook struggled to go 6-8 with a 5.08 ERA. His season was interrupted by a toe injury, which allowed him to go to the Minors to clear up mechanical issues. It was ended on Sept. 8 when a line drive by the Reds' Joey Votto cracked his left fibula.
"Bo McLaughlin, a guy I have a lot of respect for, said [Sunday] was the best he's seen me throw in two years," Cook said. "Balls were down. I was pitching to both sides of the plate effectively."
Outside of ace righty Ubaldo Jimenez (0-5, 5.86 ERA), who has battled injuries and inconsistency, the rotation has thrown well, but the loss of lefty No. 2 starter Jorge De La Rosa for the season because of a torn elbow ligament has hurt from an experience standpoint.
"I want to be here and I want to help out," Cook said. "I hope they've seen my commitment to this team. I want to be a part of something great. I think we have a great team here. We've struggled in May very badly, but we've got a great opportunity to do some wonderful things here."
The upshot of delaying Cook's return is Nicasio, who throws his fastball in the upper 90s and has a reputation for fearlessness, has a chance to show he's more than a stopgap. Another strong start could cause Tracy to juggle the rotation to make sure there's room for Nicasio.
"We want the best group of guys that we feel give us the best chance to win going forward, so he gets another opportunity as do other guys," Tracy said. "Aaron Cook is a veteran guy that has had quite a career here in Colorado. Rather than pin anything down, which is not necessary to do at this point in time, let him go out there and pitch. Depending on what we see in start No. 2 from Juan Nicasio, does it force you back to the drawing board and have you feel as though you have to realign some things because he's forcing himself in?"