Replace it when you can. But, you will find that the D seal is only sold in longer lengths, and will run you around 100 bucks.
Since the D seal just slides onto its track (also a plastic/ or vinyl piece, if water does enter it probably won't hurt anything, BUT, a quick fix...
It looks like the rip is only about 2-3 inches from the end, but up high.
Slide the seal off it's track, and reverse it. Now the rip will be at the very bottom, and any water that does enter will run out the open end.
Another possibility would be to use an older tube or tire repair patch with the rubber cement. Those patches would hold forever on tire tubes.
Steve & Cheryl + Zoey, and Ziggy, our furry kids.
2012 Forrest River Lexington 283ts
Toad, 2013 Subaru XV Crosstrek