Maclin has yet to practice this summer and his status has been in question due to the unknown nature of his illness, fueling speculation about the third-year receiver.
"I think he's going to be ready," Reid told Comcast SportsNet Philadelphia. "I guess I'm optimistic. I'm waiting to hear the results, but I'm optimistic."
Maclin, who had 70 receptions for 964 yards and 10 touchdowns last season, flew home to St. Louis on Thursday to undergo more testing to try to determine the cause of his illness.
"He has to have some tests done," Reid said. "His (surrogate) father is a doctor, so that is actually a comforting feeling. He has someone there that he is very close to, and his dad has a good pulse on things, which we encourage, so that's where he had some of the tests done."
Reid also expressed optimism over newly signed receiver Steve Smith, who had microfracture surgery on his left knee in late December. Reid wouldn't put a timetable on Smith's recovery and said the team will bring him along slowly.
"He had a small microfracture, but all of his ligaments and cartilage (are) intact," Reid said. "His knee is very stable, and he's had no swelling in his knee throughout the whole process. He's been running, and he started running routes this past week."
The Associated Press contributed to this report