Arizona manager Torey Lovullo said Tuesday that Pollock has an avulsion fracture, which occurs where a tendon or ligament attaches to the bone.
The 30-year-old Pollock leads the team with a .293 batting average, 11 home runs and 33 RBIs. He was an All-Star in 2015, but he has not played more than 112 games in a season since because of injuries.
"It's tough to lose anybody,'' Diamondbacks general manager Mike Hazen said, "but A.J., the way he was playing, it's a pretty big blow. But nobody's going to feel sorry for us. We've got to pick up and move on.''
Pollock, who becomes a free agent after this season, missed all but 12 games in 2016 after fracturing his elbow in a headfirst slide at home in the team's final exhibition game. Last year, he missed nearly two months with a groin injury.
"Look, I hurt with A.J. just like everybody else does,'' Lovullo said. "We are all aware of the season that he was having. I can't give you his gut reaction, but mine is one of empathy and understanding of what he's probably walking through right now in dealing with frustration.''
Arizona began the night with a major league-low .223 batting average and a six-game losing streak, its longest in two seasons.
Lovullo said Jarrod Dyson and Chris Owings would fill in for Pollock, who is one of the best defensive outfielders in the National League. Dyson was signed as a free agent in the offseason after playing for the Seattle Mariners. Owings, the starter Tuesday night, has become a do-everything infielder-outfielder but has not played in center since 2016.
"We feel like we have created depth here for these situations,'' Lovullo said, "and it's time for guys to step up and take advantage of opportunities until A.J. returns.''
Dyson is hitting .184 with two homers and eight RBIs. Owings is hitting .209 with two homers and nine RBIs. They are part of a teamwide slump that has affected most of the lineup. There are no plans to look outside the organization for help.
"I think at this point of the season internal is your only real option,'' Hazen said.
In the batting order Tuesday night against Milwaukee, Paul Goldschmidt dropped a spot to Pollock's cleanup position. The order should be beefed up soon by the return of third baseman Jake Lamb, who is on a rehab assignment with Class A Visalia.
Pollock was hurt when his glove hand appeared to twist awkwardly as he hit the ground in his diving attempt, with the ball sailing past him. Neither Lovullo nor Hazen would criticize Pollock for trying to make the play, regardless of the risk of injury.
"We never ask our athletes to back down from any challenge, from any play,'' Lovullo said. "That's what good teammates do.''
Hazen said that's just the way Pollock plays.
"You wouldn't expect any other way,'' Hazen said. "For guys like A.J., it doesn't matter what the score is. It doesn't matter what the situation is. He's just trying to save runs there. He's going to do that, I bet, 100 out of 100 times. And it's bad luck.''