Palmer was introduced at halftime and helped turn the match around as Chelsea won 3-2 to qualify for the final 16.
Rosenior said of his star midfielder: "There was no way Cole could have started this game and played 60, 70 or 80 minutes,’ Rosenior reflected after our victory in Naples.
"Napoli have had a difficult time with injuries, so I felt that later in the game we would be stronger, and I felt Cole’s performance was magnificent. Not just this quality, but his pressing and his running for the team.
"He's had so many games over a long period of time (in the last two seasons), so we need to look after him, and I think the way that he's managed himself for the last two weeks has been magnificent.
"Hopefully, we can now see the best of him now that he's fully fit."