D'Arcy has played just sixty minutes of rugby - as full-back during the 16-12 Celtic League loss to the Ospreys - since pulling up with a hamstring strain against Italy in the RBS Six Nations last month.
"Gordon is recovering all the time. He is just trying to build up the strength in his hamstring at the moment. He'll be training again later this afternoon but he is well able to sprint. But at this stage, and this goes for both players, I don't want to pick them if they are not fully fit," admitted Kidney.
Horgan - Europe's top try scorer with seven from Leinster's six pool games - broke his thumb in the Six Nations win over England, and has not featured in a Leinster side since their record 55-3 league win over the Dragons two months ago.
The 26-year-old Drogheda-born flyer had a free-scoring January - touching down five times - including a European hat-trick against Treviso - in four starts.