Fox felt some tingling in his right arm from the elbow to his fingers while playing catch Monday at Minute Maid Park.
"All of a sudden, I said, 'Whoa, something's not right,'" Fox said.
The right-hander has appeared in three games, totaling 3 1/3 innings, in his comeback. He had not pitched since April 2005, because of problems with his right elbow.
"To have the numbness and tingling back in there, to me, is a good sign," Fox said. "It's just the nerves are inflamed, or something around the nerve. I don't feel like I did when it was train wreck back in '05. It's a test. I'm ready for it. Mentally, I've been down this road."
Fox's approach has been to play it smart and take his time.
"I'll hit it head-on, and that's all I can do," he said. "It's nothing that will cause me to go home."
Before Monday's game, Cubs manager Lou Piniella was saying he needed to get Fox and Jon Lieber into games more than he had been. Lieber did pitch 1 2/3 innings on Monday in the Cubs' 7-2 win, but Fox wasn't available.
"It's frustrating, but I have to be smart," he said. "Like I've been saying since Day 1, it's a marathon, and I know in my mind and my heart I can pitch at this level, and I want to come back and win.
"It's not easy, but it is what it is. Am I scared like I was in '05? Heck no. I'll definitely be back."
Ascanio, whom the Cubs acquired last winter from Atlanta, was 1-0 with a 2.08 ERA and nine saves in 17 games for Triple-A Iowa, striking out 23 over 21 2/3 innings. Batters were hitting .179 against the right-hander.
Carrie Muskat is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.