Earlier in his career he did have trouble holding a place in the top 10 (besides his long hold on the #1 placement), often drifting from say, 5 or 6 to 17 or 18. His rank fluctuates often, I'm sure it has to do with his random losses and then long streak of wins.
Hewitt isn't healthy or consistent enough to hold a place in the top 30 let alone top 10 these days. He only plays his best tennis on grass and at the slams/davis cup... any other minor tournament he will either not play if he doesn't feel 100%, or he won't play at full capacity to avoid injury/he isn't as motivated in small events, thus losing to guys he should otherwise be beating.
He is definitely capable of being top 50 again, if he can perform consistently in slams - but this could be difficult being unseeded. Injuries are a huge question mark.