The England midfielder was substituted in the 71st minute of Wednesday's 1-1 draw with Genoa after scoring a first-half freekick to notch his second goal in as many games.
"He came off because of a muscle strain," coach Carlo Ancelotti was quoted as saying by Thursday's La Gazzetta dello Sport. "We'll assess (the situation) in the next few days, but it does not seem serious."
The 33-year-old joined Milan on loan from Los Angeles Galaxy for two months in a bid to stay in contention for England selection during the U.S. close season.
But his assured showings in four starts so far have led to a growing clamour for him to stay permanently and the player admitted he was torn about whether to return to the U.S. in an interview published on Wednesday.
If Beckham does not recover in time then Ronaldinho could return to the starting lineup as third-placed Milan seek to regain ground on Inter Milan after dropping eight points behind the leaders.
Ancelotti tried fielding Ronaldinho and Beckham together on the Englishman's league debut, a 2-2 draw at Roma, in a highly offensive lineup that also included Clarence Seedorf, Andrea Pirlo and fellow Brazilians Kaka and Alexandre Pato.
But the team was lopsided in attack and Ancelotti has sought to restore balance by bringing Massimo Ambrosini's steely presence into midfield and leaving out Ronaldinho, who is less inclined to chase back and did not look 100 percent fit against Roma.
Even when Ronaldinho was in good form in front of goal earlier this season, Kaka complained that he was having play deeper and tackle more to compensate for his compatriot's presence in the side.
Milan captain Paolo Maldini and midfielder Mathieu Flamini will be suspended for the match in Rome.
Champions Inter entertain struggling Torino on Sunday with morale high after they won 2-0 at Catania on Wednesday with 10 men to extend their lead over second-placed Juventus to six points.
Coach Jose Mourinho will be without suspended midfielder Sulley Muntari, who was sent off in Sicily, and Brazil forward Adriano, who will serve the second part of a three-match ban for throwing a punch in Sunday's 1-0 win over Sampdoria.
Juventus hope to have Alessandro Del Piero back for Saturday's home game against Cagliari after the captain missed Wednesday's 2-1 defeat at Udinese with a knee knock.
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