CINCINNATI -- Geoff Blum had seemed to be progressing well at extended Spring Training and D-backs manager Kirk Gibson remained cautiously optimistic about the third baseman's return, barring any setbacks.

Prior to Thursday afternoon's game against the Reds, Gibson revealed the latest news on Blum. And it isn't so optimistic.

Blum, who's on the disabled list retroactive to March 22 with right knee inflammation, will likely have surgery on his knee next week and could be out an additional six to eight weeks, if not longer.

"You don't really know until they go in there and see what they're up against," Gibson said. "He played in one game and didn't feel so well. I hope he can rehab and get back and be the guy we want him to be. We didn't see this coming. You don't always get what you want."

Arizona has been able to make do without Blum so far this season. One major reason why is the emergence of Ryan Roberts, who has reached base in eight of his nine starts and entered Thursday batting .375 in that stretch.

"We have 25 guys to play with," Gibson said. "That's what a team and an organization is all about. We'll just have to deal with it with other people."

In more positive rehab news, left-hander Zach Duke, on the DL since March 30 with a broken throwing hand, could begin throwing bullpen sessions next week when the D-backs return home.

Mora feels better, remains out of lineup

CINCINNATI -- Melvin Mora said he was feeling better on Thursday, but his left ankle still had visible swelling and he was out of the starting lineup for the finale of a three-game series against the Reds at Great American Ball Park.

Mora jammed his left foot into the first-base bag during Saturday's game against the Giants, and it's been hampering him ever since. X-rays taken earlier this week were negative.

D-backs manager Kirk Gibson said he believed Mora would be available to pinch-hit on Thursday, but he had the hot-hitting Ryan Roberts back at third base and batting second. Mora never entered Arizona's 7-4 loss, and Roberts extended his hitting streak to eight games with a 1-for-4 performance.

"He's played very well, no question about it," Gibson said of Roberts. "He's worked hard. Now the circumstances have given him an opportunity to play a lot."

Roberts riding high with hot bat

CINCINNATI -- Ryan Roberts extended his hitting streak to a career-high eight games Thursday, a stretch that includes three doubles, two homers and six RBIs and has his batting average sitting at .366. His hitting streak is the third-longest active streak in the Major Leagues, tied with the Brewers' Casey McGehee.

D-backs skipper Kirk Gibson credits Roberts' mental approach more than anything.

"He's approached every day the same," Gibson said. "He's learned to control his mind and just worry about what he's capable of doing, and not about what might happen if he doesn't."