Midfielder Brillante meekly gave the ball away in his own half, and the host capitalised to score through Radja Nainggolan.
Montella then substituted the former Newcastle Jets man seven minutes later, to cap a horror league debut for the 21-year-old.
But the Fiorentina boss said Brillante had earned his call-up to the starting XI.
"I chose him because he's done a great deal, while [Alberto] Aquilani really struggled in training this week, which was unusual," Montella told Sky Sport Italia.
"It was a tough decision, I never expected him to play 90 minutes, but he was an extra weapon in the armoury."
On his side's output as a whole, which saw them concede a second goal in second-half injury time to Gervinho, Montella said the performance was not as bad as pundits were suggesting.
"I think criticism is exaggerated," he said.
"We had a bad first half which was a little timorous by our standards and I have to work out why, but the first half-hour was very tactical with few chances for either side.
"Fiorentina were excellent in the second half and created numerous scoring opportunities, unfortunately none of them went in.
"After the break we showed that we had better stamina and consistency.
"In the first half Roma were more reactive and got to the ball first, but despite that I didn't see that many chances from them.
"If we had played the first half the way we did the second, it would've been a very different game. I don’t know what happened.
"We seemed more concerned with defending than attacking. I must've communicated my intentions badly."