Donald finished 37th at the RBC Heritage, won on Sunday by Sweden's Carl Pettersson, where the Englishman needed to finish in the top eight to retain top spot.
"Is it a concern? No," Donald, who finished 16 shots behind Pettersson on two over, said on www.skysports.com. "Obviously, I'd have liked to have played a bit more consistently this year. I built it up nicely last year through tournaments and winning a bunch."
The result allowed McIlroy, who took the week off following The Masters, to go back to the top of the pile.
He tweeted: "#1 again without touching a golf club this week.... I wish it was that easy!"