Arnautovic pushes Inter six points clear ahead of Bayern showdown
Playing in place of Marcus Thuram with coach Simone Inzaghi looking ahead to Wednesday's quarter-final second leg with Bayern, Arnautovic lashed champions Inter ahead in the 13th minute.
And the Austria international laid on a perfect sweeping pass 13 minutes later for Lautaro Martinez, who dinked home his 20th goal of the season in all competitions.
Yann Bisseck's thumping header in the 55th minute made sure Inter would collect all three points at the San Siro, after Roberto Piccoli halved the deficit for 15th-placed Cagliari shortly after half-time.
Inter now wait for the result of closest challengers Napoli's home fixture with Empoli on Monday, by which time attentions will be squarely on Bayern who they lead 2-1 following Tuesday's first leg in Germany.
"We'll go in that match with a lot of confidence even though we know that we'll be playing one of the three or four best teams in the world," assured Inzaghi.
"We've need to repeat the pace, aggression and organisation of our performance in the first leg."
Arnautovic has played a bit-part role at Inter but has had an impact in recent weeks with crucial goals, Saturday's being his fifth in his last 10 club appearances.
Inzaghi's team are gunning for a repeat of the Serie A, Champions League and Italian Cup treble they conquered under Jose Mourinho 15 years ago.
Arnautovic, 35, was on loan at Inter from FC Twente when Inter beat Bayern to win the Champions League in 2010, but he said "it was a bit like I was a fan cheering the team on".
"Obviously we believe in the treble because we're Inter, but there's a long way to go," said Arnautovic, who played just three times for Inter under Mourinho.
Earlier Venezia boosted their survival hopes by beating rock-bottom Monza 1-0 and moving to within two points of Lecce who sit just outside the relegation zone and lost 2-1 at Juventus.
Juve moved up to third on 59 points thanks to a fine strike from Teun Koopmeiners after less than two minutes and Kenan Yildiz's effort in the 33rd minute in Turin.
Igor Tudor remains unbeaten three games into his reign after replacing the sacked Thiago Motta last month and is on course to guide Juve to the Champions League.
Juve are a point ahead of Atalanta who host Bologna -- a further point back in fifth -- in Sunday's first match, and as a result are guaranteed to stay in the top four for a week.
However, Juve were again sluggish and uninspired despite dominating play, and even risked throwing away the win after Federico Baschirotto glanced home his second goal in as many games for Lecce three minutes from the end.
