Tsarukyan vs Hooker, I would have said that it is a step back for Tsarukyan, but instead I would say it is two steps back for him 
Before sudden back issues Tsarukyan was considered as a possible champion. Now he is considered a guy who will beat Topuria. And instead he fights a guy who has lost most of his biggest fights, and his recent best achievement is a win over Gamrot (a split win btw). Puelles, Turner, they are not even in top, so a win against them does not count as cool And before them Hooker had losses.
Before sudden back issues Tsarukyan was considered as a possible champion. Now he is considered a guy who will beat Topuria. And instead he fights a guy who has lost most of his biggest fights, and his recent best achievement is a win over Gamrot (a split win btw). Puelles, Turner, they are not even in top, so a win against them does not count as cool And before them Hooker had losses. Like him or not, Hooker is now ranked No: 6, so just one spot below Gaethje, who is fighting for the belt.
Obviously, Topuria Vs Tsarukyan would make more sense, but Arman gets penalised for ruining the main event and pulling out in the last minute. And since Topuria is fighting Gaethje next, Arman should have a fight too. That's much rather than just sitting and waiting. Out of the top guys, Hooker (on paper) is one of the easiest opponents Arman could get. And he's entertaining so can bring more views and therefore more money to both. Plus, since Hooker defeated Gamrot and Gamrot defeated Arman, it's a chance for Arman to have an indirect revenge and prove that his skills are better.
I am looking on this fight from different angle. Suppose Arman is the person who could have beaten Islam if he hasnt pulled off just before the fight, suppose Arman is a guy who is able to beat Topuria and became a champ. On a paper we have a potentially next champ who is risking his place in top against a "an easiest opponent he could get". On the scales are belt and chance to get injury/chance to get KOed and forget about fight against champion (Topuria) for next half a year (doctors wont let you fight for 6 months if you are KOed). Possibly he could waste his peak on nothing, when he should spend it on defending belt