The Chelsea midfielder has suffered a recurrence of his injury, ruling him out for matches against Manchester United and Shakhtar Donetsk, giving young Marko Marin a chance.
Di Matteo was facing the press ahead of Sunday's encounter with Manchester United, and he conceded that Lampard will miss the game at Stamford Bridge. The England midfielder aggravated the pre-existing injury in Tuesday's 2-1 UEFA Champions League defeat to Shakhtar Donetsk and will miss the return leg, in addition to the United clash.
"He is going to be out for a few weeks," Di Matteo said. "He's got a calf problem, so we will miss him and it's a big loss for us. Apart from him everybody else is fit. Marko Marin might be in the squad for Sunday."
The 34-year-old Lampard had already been forced to sit out England's World Cup Qualifiers against San Marino and Poland in a bid to recover quickly from the injury, but he only lasted 15 minutes against Shakhtar in Ukraine before being withdrawn.
Lampard has played in seven Premier League matches this season, scoring three goals so far.