The Tangerines take on Danny Lennon's men knowing they are approaching the business end of the season and could do with a win to boost their top-six hopes.
United ground out a hard-fought 2-1 Scottish Cup quarter-final victory over Dundee at Dens Park last weekend and McNamara would take more of the same in Paisley this weekend.
He said: "It's all about winning at this stage in the season. That is what it comes down to and we know that.
"It wasn't the best game to watch last Sunday but the main thing was getting into the next round.
"We'll have to be prepared to dig deep again against St Mirren as they are a difficult team to play against and we have had them watched.
"We always try to play football and hopefully we can do that but the bottom line is trying to get the three points."