Merrick said it was particularly disappointing to see Sigmund pull up lame on Wednesday at Newtown Park.
"We've been bringing him along very slowly and very progressively but without overdoing it. But as soon as he went into a game, it pulled again. It looks like it's going to be a long-term injury - I feel for Ben."
Although Merrick isn't expecting a quick recovery for the 34-year-old defender, who's retiring at the end of the season, he's still optimistic Sigmund hasn't played his last game for the Phoenix.
"We've got another half season to go through, another 14 games," Merrick said. "I think he'll play before the end of the season. We told him he's got to play two more games before he gets his bonus.
Merrick is confident he has the players to cover, pointing to young Australian Dylan Fox as a likely starter in the back four.
"He's been training really well - he's quick, he's technically good and he's got good vision."
Albert Reira returns from his one-match suspension to boost Wellington's midfield, where Merrick is looking at two holding midfielders, with extra attacking options from the likes of New Zealand under-20 rep Joel Stevens or under-17 youngster James McGarry.
"Up front, we've still got Mikey McGlinchey, Blake Powell, Kwabena Appiah, and Logan Rogerson's looking really good.
"You know what it's like - you give someone an opportunity and, all of a sudden, you find a very, very good player."
-AAP