Grimlock ♥ Ultra Magnus

After all, the spell he put in Zelma could destroy her: if Loki finds out where the spell is, he could try to remove it, which would most likely tear her soul apart.   Loki also kills his talking dog, so there's that big thing. 

So he goes to the person he saved, that he promised he would never ask for help again - and hopes against hope that they'll take up Doctor Strange's cause as their own.