It's not whether or not Aizen is a serious threat for the Zero squad to get involved.
WARNING THIS HAS SPOILERS FOR THOSE NOT CAUGHT UP!
Reading the manga, when Soi-Fon angrily bashes on them why they didn't get involved, the Zero squad states that the things going on Soul Society is the responsibility of the Gotei 13 and that the Zero squad is for the protection of the Royal Family.
The manga also goes on to reveal the fact that IF Aizen did construct the Key, the zero squad would have intervened. They didn't because at that time he wasn't really a threat to the Royal Family.
However, I do believe that he is also much weaker than the Zero squad (Note this is MY opinion). The reason why is first of all, Juha Bacha, the leader of the new Quincies says "We better fall back before the Zero Squad comes." Juha Bacha who easily defeated the CC by stealing his bankai seems to be concerned with the Zero Squad. Also, in a flashback moment when he's talking to the dead CC about how weak the Gotei 13 got, there are shadows that look very similar to Unohana and the current Members of the Zero Squad. This means that they were part of the time when Gotei 13 was bloodthirsty and unstoppable which makes me think that they were around (a little weaker) than the CC. Aizen only defeated the CC with the help of Wonderweiss who absorbed the CC's powers. If the Zero squad is around that level, Aizen would have been completely dominated.