But two months later, things have changed. Jenkins finds himself on the big league roster, and McGuire may not be far behind, according to Double-A pitching coach Tom Signore.
"I think they are ready. I think they can handle it," Signore said.
Jenkins, who was called up on Sunday after the club's latest midseason makeover, will be used as a long reliever after serving as a starter for his entire Minor League career.
Had the pair, both first-round Draft picks, been pitching as effectively in June as they are now, perhaps they could have filled the void after Toronto lost three-fifths of its rotation in the span of a week.
But much like the Blue Jays' season, there has been a lot of trial and error for the two young right-handers.
Both entered Spring Training on the radar as possible, if not likely, rotation stopgaps had anything catastrophic happened to the pitching staff -- which it did. But Jenkins, taken 20th overall in the 2009 First-Year Player Draft, and McGuire, 11th in 2010, both scuffled to begin their seasons, and Signore believes the pressure of their stature as high picks and players who could move quickly through the system affected their performance.
"I don't think there is any question, as a first-round Draft pick, [that] you carry that around on your shoulders," Signore said. "I think they carry that burden and try to live up to that. I think a lot of players get a little bit tight and feel they have to produce or think they will be considered a disappointment."
Jenkins and McGuire both watched as numerous teammates on the Fisher Cats took advantage of the injuries to the big league club. Relievers Evan Crawford, Aaron Loup and Sam Dyson all donned a Blue Jays uniform before Jenkins did, as well as a number of others from Triple-A Las Vegas.
But the two kept plugging away, and Signore started to see improvement, especially in their changeups, which they both began throwing with more conviction.
"They are both starting to get it -- both developed pretty good changeups," Signore said. "It's a 'feel' pitch. You have to trust it and throw it with the same arm speed [as the fastball].
"If they both trust their changeups, they will be fine."
Much of the credit can be given to roving pitching instructor Dane Johnson, who has been instrumental in teaching the changeup to the young arms in the lower levels of the organization.
Signore also noted that Jenkins and McGuire are just 24 and 23 years old, respectively, and this year embarked on their first full season in Double-A after beginning 2011 at Class A Dunedin. To him, any thinking that their development has been moving slowly is nearsighted.
"He takes incredible care of himself and works very hard," Signore said about Jenkins, who logged three consecutive quality starts and seven in his past 10 before earning the promotion. "He has stayed the course, and it's all paying off for him now."
Signore describes Jenkins as a ground-ball pitcher who induces a lot of weak contact with a sinker that gets underneath opponents' bats.
Jenkins' fastball sits at 90 mph or 91 mph, and he can run it up to 93, a number he may be able to touch more in shorter relief stints with Toronto. Signore adds that Jenkins also has a viable slider to complement his heater and that his improved changeup mimics what his sinking fastball does by showing very similar fade.
At times, Jenkins can come out of his delivery, but Signore says that's something the young pitcher recognizes and is quick to correct during a game when necessary.
"I think he is in a great place right now. I think he has a very good feel as an athlete and is a very good self-teacher," Signore said. "He has very good aptitude."
As with Jenkins, some mechanical issues slowed McGuire earlier in the season. He had a tendency to kick rather than lift his front leg, which opened his delivery too much. Once that was tightened and McGuire could keep his side upright, Signore believes, he took a step forward.
McGuire feels that he needs to command his 89-to-93-mph fastball better, but Signore says he has gotten better at it and can routinely pound it to his glove side.
"If you can hit that spot, the rest of the repertoire is easy to handle," Signore said.
McGuire can locate his slider -- his best pitch -- on both sides of the plate, and generates weakly hit balls off it when bats don't miss entirely.
After logging four quality starts over his first 15 games of the season, McGuire has four in eight since the beginning of July and has done enough to warrant consideration for a September callup, in Signore's opinion.
"Deck has improved tremendously over the year," Signore said. "It's not how we start the season but where we are at the finish, and I think that product is going to look pretty good.
"It would be nice for him to get a taste of [the big leagues]."
Though McGuire's focus is on New Hampshire, he feels he's better prepared for the Majors thanks to the advice he has received from the teammates who have been summoned to Toronto.
"Sometimes you put the Major League level on a pedestal, almost like it's this unbeatable monster," McGuire said. "To have those guys come back and share their experience, both good and bad, it has been kind of a light at the end of the tunnel for all of us."
Chris Toman is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.