But they were unable to build on their 12-0 advantage they held at the break and faced a late onslaught from the visitors during the 12-7 win.
"We're happy to get the victory but a little bit disappointed to be honest," Evans told Ospreys' website.
"We left a couple of tries out there, and perhaps another bonus point. We'll take the victory but there's plenty to work on.
"It was basic errors. We were getting into their half, into their 22, and we were spilling the ball. There's a lot that we need to pick up on next week before we play Leinster."
Evans, who was named man of the match, added: "Our defence held strong at the end. There was a little bit of relief as we needed to get the victory but disappointment at the end in how we got the victory.
"We need to keep this run going now. Hopefully the momentum is building. It's getting tight in the table now, from the top reaching right down to mid table.
"We're happy to get the win but we have to keep that momentum going against Leinster. Every game is a big game for us now."
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