Rea, drafted from Honda's World Superbike team to replace the injured Stoner, got his first taste of 1000cc MotoGP machinery on the Monday immediately after the Czech Grand Prix.
He is expected to compete in the forthcoming Misano and Aragon races in Stoner's absence.
Rea said adapting to Bridgestone rubber - WSBK uses spec Pirelli tyres – was the main physical obstacle, but admitted he also chose to build up his confidence gradually rather than risking too much, too early.
"My first impressions are that the bike isn't that difficult, it's the tyres and the way they work that I'm finding hard," he explained.
"I'm struggling to finish off the corner which is definitely a confidence issue as I just need to load the front and find that confidence but it's my first day and I don't want to find that limit, or risk overstepping it just yet."
Rea, whose best time of 1m59.595s was fractions shy of his WSBK pace at the Czech circuit in July, said his approach had been to learn about the bike rather than to chase laptimes.
"I don't underestimate how tough a job it is, I just need some time and I don't feel pressure to perform," he said. "It's a learning process with the team and people around me.
"It's great to be able to see Dani's data and learn where to go fast, that's a big help.
"It [was] a great day, one of the best of my life. I've been a Honda rider all my career and now to ride the best Honda machine in the world has been a dream come true".
Rea added: "It's an opportunity of a lifetime for me and I'm thankful to Honda for asking me to be here, although I'm gutted for the guys in the team and obviously for Casey as he is the guy that should be here I wish him a fast recovery."