CHICAGO -- It was evident on Monday night that something was not right with Cubs closer Pedro Strop. The reliever looked uncomfortable on the mound and was laboring mightily to find the strike zone.

The explanation arrived on Wednesday, when the Cubs placed Strop on the 10-day injured list with a left hamstring injury and activated lefty Mike Montgomery (left lat) from the IL. Strop said that he underwent an MRI exam on Wednesday that revealed a Grade 2 strain and noted that he would get treatment for three or four days before determining the next step in his rehab.

"It is frustrating, especially me, I take everything very personal," said Strop, who does not have an established timetable for return. "I take my job, my team, wins, losses, I take it personal. So, for me to be sidelined at this point when we're playing so well, it is kind of a challenge."

There are also added challenges now for both manager Joe Maddon and Cubs president of baseball operations Theo Epstein. For Maddon, his task is now to determine which of his relievers can serve as the end point on a nightly basis. For Epstein, the Strop injury -- coupled with the recent setback to reliever Brandon Morrow, who is on the 60-day IL with a right elbow issue and out until at least July -- might increase the need to search for external bullpen help.

Epstein said the Cubs' front office would be on the hunt for potential relief help, even without the development with Strop.

"It doesn't really change anything for us," Epstein said. "We recognize that this is a year where we're going to be constantly on the lookout to make adjustments to the bullpen, to try to put the right relievers in the right position to be successful, that we'll have to tweak and adjust over the course of the year as we go. And that we'll receive a lot of help both internally and probably at some point from outside the organization.

"We have been in, and will continue to be in an aggressive mindset with respect to the bullpen. I think the results thus far have been outstanding, but that doesn't mean we become passive or assume that that's going to continue going forward. We're going to be challenged in that area throughout the course of the year."

Entering Wednesday, the Cubs' relief corps had a collective 2.29 ERA with a .592 opponents' OPS across 24 games dating back to April 7. That stretch comes after the group turned in a 9.51 ERA with a 1.041 opponents' OPS in the first eight games of the season. Without Strop in the picture, Brandon Kintzler, Steve Cishek, Brad Brach, Carl Edwards Jr., Tyler Chatwood, Kyle Ryan or Montgomery could see save chances if the situation makes sense.

"You've got to save somebody for that moment," Maddon said. "It'll be all the guys. It'll be everybody. It'll be Cishek. It'll be Brach. It'll be Kintzler. It'll be Carl. It'll be Kyle Ryan. I think Montgomery's capable. It's everybody. Chatwood. I mean, listen, I'm really curious about this, too. If Chatty goes out there and pounds the strike zone, there's a lot that he can do good for us.

"It's wide open. It is the wild, wild west. I'm not going in with any preconceived ideas except: 'Who are our guys for tonight?' And then I've got to build out the plan from that pregame and then game in progress, make the adjustments."

Worth noting

• Cubs president of business operations Crane Kenney issued a statement on Wednesday indicating that the organization has identified the fan who made an offensive hand gesture during the TV broadcast of Tuesday's game against the Marlins. Kenney said the act was in violation of the team's Guest Code of Conduct and the fan will be notified that he is no longer permitted on the grounds of Wrigley Field or other ticketed areas indefinitely.

"The incident last night is truly disgusting," Epstein said. "It gave me shivers to watch that, that that would take place at Wrigley Field. ... We've made clear how egregious and unacceptable that behavior is and there's no place for it in our society, in baseball and there's certainly no place at Wrigley Field. The person responsible for that gesture will never be welcome back at Wrigley Field. I think it's important to have a strong response, to send a message that this is a place of inclusion."

• Epstein declined to disclose any further details about the situation involving veteran utility man Ben Zobrist, who was placed on the restricted list on Wednesday in order to tend to a personal matter. There is no established timetable for Zobrist's return to the Cubs.

"The only thing appropriate for me to communicate is that he's requested a leave of absence from the team for personal family reasons," Epstein said. "That leave was granted, and he'll be away from the team for a period of time, and that, if and when it's appropriate for him to provide further detail or clarification, he will."

• Utility man Daniel Descalso remained out of the starting lineup on Wednesday, marking the fourth game in a row he has been on the bench due to left ankle soreness. Epstein said it could still be a couple days before Descalso (available as a pinch-hitter) is deemed ready to resume playing a full nine innings.