Portugal captain Cristiano Ronaldo slammed Luxembourg's pitch as a "potato field" and also said he had not been fully fit in recent matches after the European champions won 2-0 on Sunday to qualify for Euro 2020.

Ronaldo scored his 99th Portugal goal late in the game, having notched a hat-trick in a 6-0 win over Lithuania three days earlier, but it was far from a vintage performance on a pitch full of divots at the Josy Barthel stadium.

"It's difficult to play on these pitches -- it was a potato field, I don't know how teams of our level are supposed to play on these pitches," he told reporters.

"It wasn't a good spectacle but we did our job... I'm looking forward to my fifth European Championship."

The 34-year-old had joined up with the Portugal squad amid speculation over his fitness and attitude following his angry reaction to being substituted during Juventus' 1-0 win over AC Milan the previous Sunday.

Juve coach Maurizio Sarri then said that Ronaldo had been struggling with a knee problem for some time.

"In the last three weeks, I have been playing with limitations," Ronaldo told a group of reporters outside Portugal's dressing-room door on Sunday. "But there was no controversy -- you [the media] created that.

"I tried to help Juventus. Nobody likes to be substituted but I understood because I wasn't in such good shape," he added.

Ronaldo said he was also "not 100 percent" in the two Portugal matches.

"I sacrificed myself for the team," he said. "We could have missed out [on Euro 2020]. Thankfully, I haven't been injured much in my career but it can happen."

"It's a pain that prevents me from being 100 percent, but I always try to play. They created a big controversy where it didn't need to exist. I'll be back to 100 percent soon."

Portugal finished second in Group B with 17 points, three ahead of Serbia who drew 2-2 with Ukraine.

Ronaldo would become only the second player to score 100 goals by reaching that milestone.

"The 100th goal will come naturally," he said. "Records are there to be beaten and I am going to beat them."