Harrison threw three pitches in the top of the fourth inning before feeling the numbness. He will be further examined and most likely will see Dr. Keith Meister in Arlington, Texas.
Harrison threw 60 pitches, just shy of the 65 he was expected to throw in his third rehab start. He allowed three earned runs and three hits in three innings.
"I felt strong from the get-go," Harrison said. "The ball was coming out of my hand good. Except for having to come out of the game as a precaution, a little bit of tingling, other than that, I just had some long innings, deep pitch counts. I threw a lot of pitches in those three innings."
Harrison, the Rangers' Opening Day starter, made two starts to begin the season before an inflamed nerve in his lower back required two surgeries. Harrison won 18 games last season, tying Kenny Rogers for the most wins in a season by a Rangers left-hander.
The Rangers are hoping Harrison can return in a few weeks and bolster their rotation.
Also, designated hitter Lance Berkman (knee) will begin a rehab assignment with Triple-A Round Rock on Saturday after making it through Friday's workout with no issues.