The Reds won 4-1 to reach the semis 6-1 on aggregate.
Henderson said, "They are a good a team, they made it intense and the lads dug in and defended really well. They had some half chances. You could see the effort that we put in. I was warming up when Sadio [Mane] scored and I thought it was a goal. I think I was the only one.
"Thankfully it was given and it gave us a real boost coming in at half time.
"When I came on I tried to keep the ball and be sensible. Joe [Gomez] made a great run [for Roberto Firmino's goal], I just tried to hit an area and it's a great finish from Bobby.
"The scoreline flattered us a little bit. Porto had a real good go. Even when we went in front it was a real good atmosphere."