"In that last 15 minutes, they started throwing the ball around a bit and they stretched us in defence," he said. "There were a couple of tired bodies out there but I expect them to do that from the word go on Saturday.
"If the weather allows it, I think they're going to stretch us a bit and play a little bit more rugby. But, at the end of the day, when you play more rugby there's a chance that you can make more mistakes.
"Our defence has got to be right up there again and we have to play a bit better. At some stages we played some great rugby and at others we were quite poor.
"We gave away a lot of soft turnovers and if we can make sure we don't give easy ball away, we'll be right back in it."
Ulster Rugby - who followed Bath Rugby as tournament champions in 1999 - had to come from behind to get the four points at Ravenhill and Springbok lock Muller relished the fight back by the home side.
"That probably rates right up there with one of my most enjoyable games of the year," he said. "It was great fun, it was really tough, it was physical and it was quick.
"To be 15-3 down and to come back and get the win at the end was a really good effort. I really enjoyed it - it was really tough rugby and that's what it's all about.
"When the chips are down, like they were in that first half, you really need the team to stand up and to get that win was a very special effort.
"At times we played some excellent rugby. I think they gave away three penalties two metres away from their own try line and if they hadn't, we probably would have scored there.
"But it was very important to keep that scoreboard ticking over and Ian (Humphreys) did that really well. When you get penalties, you've got to take the points and, credit to our kicker, I thought he was outstanding."
Saturday's contest will be the fourth tournament meeting between the teams with the score now standing at 3-0 to the Irish province, who won 28-10 at The Rec.
"It's a huge game for us," said head coach Brian McLaughlin
"Last year we got our first win on English soil, but that's history. It's another game for us and the importance for us moving forward in this competition is immense.
"We have to go there and get a win. Last year it was smashing to win and there's no doubt that it gave us great confidence for the rest of the season.
"But this is what it's all about now and we need to make sure we do a job and come away with the points."