"It's marginal at the moment," Smith said. "If it was anything else I would just play no worries, but with my hammies and my history I have to act with caution.
"I'm not as quick as I used to be but I try to make up for that with smarts."
Smith's experience would be a huge asset to a young Birkenhead side. He won the Chatham Cup as a 17-year old with Napier City Rovers - partnering Shane Smeltz up front - has played for Toronto FC and Seattle Sounders in the MLS and Swedish outfit Ljungskile SK.
Birkenhead face a Central side with Auckland City additions. ASB Premiership golden boot winner Emiliano Tade will lead the line. Korean import Dae Wook Kim and Japanese left back Takuya Iwata add quality.
"We've played them already, when they didn't have their national league boys back and they beat us then at home, so we know the quality they are going to bring. But we're also quietly confident that we have the players to test them," Smith said.
"Last weekend there were periods where we were class and played Three Kings off the park."
Other key third-round matchups include Lower Hutt City hosting Miramar Rangers, and Waitakere City FC hosting Melville United in a rematch of the 2013 semifinal.