Weeks, who had a similar injury to his right wrist in 2006, hurt himself while striking out Sunday in St. Louis. He left Busch Stadium wearing a brace and underwent an MRI exam Monday in Arizona that revealed the extent of the damage.
"We're going to miss the guy," manager Ken Macha said before Monday night's game against the St. Louis Cardinals. "He was having a tremendous year, a great start. But nobody's going to feel sorry for you."
The Brewers had won 11 of 13 and led the division by 1½ games, recovering from a 4-9 start with Weeks as a catalyst.
"I was enjoying myself watching him play," Macha said. "This is one tough guy, a grinder, very intense. It's a shame."
The oft-injured Weeks was hitting .272 with nine home runs and 24 RBIs. His homer total ranked second among NL second basemen behind Philadelphia's Chase Utley, who had 10.
General manager Doug Melvin said Weeks had a torn sheath, which keeps the tendon in place in his wrist, and will likely have surgery this week. Recovery time is four to six months and surgery is likely on Wednesday.
"Rickie was making progress. He was showing he could be the kind of player we all thought he could be," Doug Melvin said.
"You feel bad for the team, but you feel bad for Rickie, too," he said. "He put so much work into it."
Craig Counsell filled both of Weeks' roles, second base and leadoff, on Monday, and was hitting .339 with a homer and seven RBIs. Macha said the 38-year-old Counsell is not likely to be the everyday second baseman because of his versatility off the bench, plus noted he had some knee problems in spring training. Macha said rookie Casey McGehee would probably get two starts against left-handers in Houston later this week.
Counsell also has been Macha's top pinch-hit threat, going 2-for-7.
"He's an extremely valuable piece to the puzzle and I'm not going to wear him out," Macha said. "He's our backup shortstop, third baseman, he backs up everywhere.
"I don't want to get this guy broken down, and he's a little older, too."
Counsell batted leadoff for the second time on Monday. Corey Hart has done it twice. Macha said he wasn't ready to move Hart to the top of the lineup yet because he's struggled a bit at the plate.
The Brewers went with a 24-man roster on Monday, and Macha said the team was likely to add an infielder on Tuesday before beginning a three-game series at Houston.
Melvin noted Weeks was on a pace to score 100 runs and have 100 RBIs this year. Weeks, the No. 2 pick in the 2003 draft, had a career-high 46 RBIs last season, helping the wild card Brewers reach the playoffs for the first time since 1982.
Weeks was hurt on his final swing in the first inning Sunday.
"I just felt a little pressure, a little tug on it," he said after the 8-2 win over the Cardinals.
Weeks missed the last two months of the 2006 season after undergoing surgery for a tendon injury to his right wrist. He was examined by Dr. Don Sheridan, who performed that operation as well as one on his left thumb in 2005.