I noticed a 1.5cm slit in my rear tire today - I don’t think it went all the way through and definitely didn’t puncture the tube.

Coincidentally, I have a new wheelset and new tires being shipped to me now but I have a 50mi ride on Saturday. To avoid a blowout, I applied a small tube patch to the outside of the tire. Thoughts? Will it help, is it useless, etc?