I agree. Michael never would have given Zasa a pass after that attack, but he probably would have seen him as a means to an end: try to find out who the real traitor was. Just before his diabetic attack, he was telling Neri, Connie and Vincent: "Get word to Zasa that I respect what he's done, that the old order has to pass" (or something to that effect). No doubt it was an effort to set him up to find our more, then kill Zasa.
There's a parallel in GFII: Michael knew with near-certainty that Roth was behind the Tahoe attack, but he went along with Roth--even traveling to Havana, where he was in mortal danger--because, as he told Frankie, he wanted to find out who the traitor was in his family.