Manager Dusty Baker said the team will decide in the next couple of days whether to put Rolen on the 15-day disabled list.
"Life's not pleasant without Scottie in there," Baker said.
The 35-year-old Rolen played in his sixth All-Star game last week. He's batting .287 with 17 homers and 57 RBIs, though his production had slipped lately. Rolen is in an 0-for-13 slump that is his longest of the season.
Rolen missed two games before the All-Star break with a sore back. During a 3-2 win over Colorado on Friday night, he was thrown out at third while tagging on a fly ball in the eighth inning. Rolen felt a burning sensation in his right hamstring while tagging up.
He went home that night and came down with the flu, which has gone through his family. He missed the last two days because of the illness. Doctors decided to treat the hamstring with an injection on Monday, hoping it will heal quickly. The team hadn't mentioned the hamstring problem until after Rolen got the treatment.
"We'll give it today and tomorrow, see how it recovers and take it from there," Rolen said.
The Reds had been in first place in the NL Central for the past 18 days before a 1-0 loss to the Rockies dropped them a half-game behind the Cardinals on Sunday. The Reds and Cardinals are the only teams in the division with winning records.
General manager Walt Jocketty traded for Rolen last July, intent on bringing veteran leadership to a young team. The Reds went 26-12 with him in the starting lineup last season, and are 45-31 with him there this year. They would like to avoid putting him on the disabled list, but don't want to play with essentially a 24-man roster for long.
"You see what our record is when he's in the lineup and when he's not," Baker said. "It puts you in a Catch-22 situation where you're operating short, trying not to use him unless it's an emergency. So we've got to make a decision here in next day or so. It depends on how he's responding to treatment."
Rolen could go on the 15-day disabled list retroactive to Saturday, reducing the time he'd have to miss. If the Reds use him in a game again, he would have to miss a full 15 days.
Rolen said he spent the last two days vomiting.
"I'm not going to be eating pizza and hot dogs in the next day or two," he said. "Still oatmeal and bland chicken. But I feel a lot better.
"Yesterday I was dragging. The day before, I was dead. The door prize for hurting my hamstring — not only do you hurt your hamstring, but you're going to throw up all night. I was limping to the toilet at 5 a.m."