Furudate-sensei drew this to promote the release of Vol 28 of the manga.
Futakuchi: Haikyuu!! Vol 28 is now on sale!! I’m on the (spine) cover, I’m on the (spine) cover~ [TL note: Spine is written extremely small in the text, so I’m putting it in brackets here] Kamasaki: Futakuchi… Haaa… Futakuchi: What?! OIkawa: Futakuchi-kun… SFX: *rustle* Oikawa: Come over? Futakuchi: …Oh, it’s fine. Please do support Haikyuu!! Vol 28. -Signed Furudate Haruichi