By his own high standards, McIlroy has endured a disappointing 18 months but the Northern Irishman was able to register his first tournament victory since September 2016 with a three-shot win at Bay Hill.
Spieth has suggested that he was not surprised by McIlroy's triumph in Florida, and says that the 28-year-old can only have benefited from a break from the sport at the back end of 2017.
The world number four is quoted by The Independent as saying: "Whether he won last week or not, he's always a force. He just needs to be healthy, and I think most of last season, it was a struggle for him.
"So just being rested, healthy and on the right path meant this year and going forward, Rory is Rory. And so he should always be a favourite at any event."
Both McIlroy and Spieth are competing at the Dell Technologies Match Play event in Texas this week as they continue their preparations for The Masters.