The result gave Jose Mourinho's side a seven-point lead over Juventus, who face Siena on Sunday, at the top of Serie A.
Inter needed Swedish striker Zlatan Ibrahimovic's 77th minute strike, his 11th of the season, to level Robert Acquafresca's 65th-minute opener for Cagliari.
The champions, who were chasing a ninth successive win, welcomed Luis Figo back for his first start since September, but scoring opportunities were scarce in the first half with Javier Zanetti wasting the best openings.
Aquafresca made them pay when he fired in past goalkeeper Julio Cesar after a long punt forward while Davide Biondini wasted a great opening to make it 2-0 in the 71st minute.
Ibrahimovic saved Inter's blushes when he found the target, but much of the hard of the work in the build-up had been done by Argentine international striker Hernan Crespo, who had only been on the pitch for 10 minutes after replacing Walter Samuel.
"It was not a positive first half and we played our best football when Cagliari scored," said Mourinho.
"Cagliari did well and threatened on the counter-attack. It was a bit of a strange match."
"Beckham is ready to play, but he's coming out of two months of inactivity, so it's difficult to name him as a starter for the match against Roma," said the Milan chief.
"He will play if he is in the right condition and if we consider he can give a positive contribution to the match. Beckham is a great professional, very committed and we are all happy with him, even if he will only stay at Milan for two months."
The ex-Manchester United and Real Madrid midfielder was "ready to help out at Milan, we are happy he's with us even if he'll only be staying two months."
"If he then wants to stay with us, that would suit us very nicely, but his contract with the club is clear and I think he'll respect that."
Beckham, who is due to return to LA Galaxy in March, made his Milan debut in midweek when he played the first half of a friendly in Dubai against German side Hamburg.
In Saturday's other match, Genoa boosted their chances of a place in Europe next season with a 3-0 home win over Torino with Giuseppe Biava, Bosko Jankovic and Thiago Motta on target.