Over his last six appearances, Bell has a 17.36 ERA and has been on the receiving end of some boos from the crowd at Chase Field.
"I think he's done better than people give him credit for," Gibson said. "I think it's sad that people boo him. He's won five games for us. He's saved 15 games for us."
Gibson appeared to blame the media for the fact that Bell has been booed.
"I know why they boo him, because a lot's been written about him and talked about him," Gibson said. "Believe me, he wants to do better, we want him to do better, but let's not discount what he has done. I don't think that's right."
Steve Gilbert is a reporter for MLB.com. Read his blog, Inside the D-backs, and follow him on Twitter @SteveGilbertMLB. This story was not subject to the approval of Major League Baseball or its clubs.