Iannone injured his left shoulder during testing in Tuscany recently.
The Italian posted a time of 1'47.940 to finish just behind ESP Movistar Yamaha's Jorge Lorenzo in third place.
What makes his fourth fastest time so impressive is that Iannone is struggling with his injury but he was still able to finish with such a competitive time.
He tells motogp.com that pain is not a problem but he is struggling to move his left shoulder.
"I had hoped to be in a better condition but instead I am having to make a big effort on this track, more than I expected.
"It's not so much a question of pain; it's more that I need too much strength, which in this moment I simply don't have.
"I am not able to move my left shoulder the way I want to and so I'm losing a lot in the changes in direction, and there are a lot of them here at Mugello," he said.
Iannone said that despite his struggles he was happy with his bike.
"I have to grit my teeth and do my best. As for the bike, I am quite pleased; we knew that the GP15 would be competitive here.
"I'm just sorry I'm not in the best condition to be able to make the best of its performance," he added.