The penalty was awarded when Plzen defender Marian Cisovsky handled the ball inside the area while attempting to mark Ibrahimovic. Cassano then scored with a delicate chip over the goalkeeper.
"It was important to win and get the three points and follow up the match in Barcelona," Milan coach Massimiliano Allegri said. In London, new signings Alex Oxlade-Chamberlain and Andre Santos provided Arsenal goals.
The 18-year-old Oxlade-Chamberlain netted just eight minutes into his European debut, slotting a shot between the legs of Olof Mellberg and to the right of goalkeeper Franco Costanzo.
Andre Santos made it 2-0 in the 20th minute, with Marouane Chamakh returning the ball into his path after an initial cross and sending a low shot inside the near post.
But Arsenal's much-maligned defense soon showed its frailties once again, letting David Fuster pull a goal back with a powerful header when left completely unmarked after a corner.
Arsenal is two points behind Marseille in Group F after Andre Ayew's double and Loic Remy's strike secured the French side''s win over Dortmund.
Chelsea tops Group E despite Roberto Soldado''s late penalty salvaging a draw for Valencia to cancel out Frank Lampard's goal.
Former Chelsea midfielder Michael Ballack scored in Bayer Leverkusen's 2-0 victory over Genk to go a point behind his old club.
In Ukraine, Jadson equalized for Shakhtar Donetsk three minutes after Ivan Trickovski scored for Cypriot visitor APOEL, which has a one-point lead over Zenit in Group G.
Zenit went ahead of Porto on goal difference after Roman Shirokov scored two goals and also set up Danny's tap-in. The Portuguese champions had led in Russia when James Rodriguez scored after 10 minutes.
AP