But Duminy said it was just an off day as captain Sachin Tendulkar's 42-ball 69 could not save the day for his side.
He insisted that Mumbai would roar back into the tournament in their next match against South Australia Redbacks in Durban on September 14.
"It is the first game of the tournament and obviously we would have loved to start the tournament on a winning note. But we are not yet out. There are three more games to go and we definitely have the side to make a comeback," the left-handed South African batsman told reporters at the post-match press conference.
"He (Tendulkar) played exceptionally well but in the end cricket is a team game and all of us needed to chip in," Duminy said.
Duminy said fielding remains a concern for the Mumbai team and lauded the Lions bowlers, especially Shane Burger who accounted for the crucial wickets of Tendulkar and dangerman Keiron Pollard, for their exemplary display at the death overs.
"I think we played pretty well but our fielding was not upto the mark and we need to address the issue in the coming games. But hats off to Lions bowlers. They bowled brilliantly at the end," Duminy said.