Center fielder Carlos Beltran already is expected to miss the first month of the season following right knee surgery Jan. 13. Reyes appears likely to also start the season on the disabled list.
"It doesn't look good right now," Mets general manager Omar Minaya said Thursday. "We will have to prepare for that."
Reyes has been diagnosed with an overactive thyroid, and his levels were elevated after he exercised Monday and Tuesday. While he initially thought he might be able to return after a few days, he will have to stop working out and will remain at home.
"Jose is obviously a little bit disappointed that it's going to be a matter of weeks as opposed to days, but it's a completely, treatable, curable situation," his agent, Peter Greenberg, said. "I think we all view it as good news."
Doctors detected an abnormality during his physical, and Reyes was taken out of the lineup March 4 for additional medical tests. Reyes was told not to raise his heart rate and to refrain from eating seafood, which has iodine. He will be tested weekly.
Minaya and Greenberg said the thyroid should return to normal with rest and diet.
"There is no medication for this," Minaya said.
The cause of the elevated thyroid wasn't clear.
"We heard it could be something that could have been dietary or it could have been a virus," Minaya said.
Reyes, Beltran and first baseman Carlos Delgado missed long stretches last year as the Mets slumped to a 70-92 record and fourth place in the NL East. Delgado became a free agent and had hip surgery again last month.