The best replacement for thumb is pacifier.

The best alternatives to thumb are rated as following:
★★★★★  pacifier
★★★★  finger trackball
★★★  arm
★★★  finger
★★  twist throttle
★★  trigger release
★  dummy
★  soother
★  wrist release
★  thumb
★  broken
★  sprain
  pinch
  purple prose
  broken thumb
  curved thumb
  sprained
  streamer
  paci
  jammed thumb
  regular thumb
  jammed
  short thumb

Your alternative for thumb is not listed? Suggest!
Speaking about thumb, one probably shouldn't forget about middle finger.

Other search terms related to thumb and their ratings are:
★★★★★ middle finger
★ splint
You have in mind a companion for thumb and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of thumb in all years. Click on graph for more.
thumb
pacifier
finger trackball