Martin decided to take off the teen, despite his run of form since the start of the season.
But he explained: "Yeah, Tyler was tactical. He looked knackered. So, we spoke about this on Thursday. He's an 18 year old lad. The level of intensity has gone through the roof for him compared to what he's used to.
"He's playing against the best team in the country, in Europe, and it just wasn't quite his day. Defensively, we needed a bit more help with Adam (Lallana), who knows and understands the game a bit more.
"And also, we felt Adam's runs in behind (would help) because he's one of the best at it. We felt it could really hurt them and it did on a few occasions. So, it was just a tactical one. Tyler's been amazing. He'll be fine. He'll learn a lot from today."