In Defenders #17, Hulk gets mad at his teammates and leaps off in a huff. In MTU #27, he gets tricked by the Chameleon into helping him free an old friend of his from prison; later in the story the friend sacrifices his life for Chameleon, and Hulk learns a lesson about friendship. He then hears on a police car radio about the Defenders' fight with the Wrecking Crew and decides to go help them. In Defenders #18 Hulk shows up for the fight, with appropriate footnote directing the reader to MTU #27.

I'm not saying the Defenders issues can't be read and enjoyed without the MTU issue; but it'd be an appropriate addition.