Merino was in the side that was beaten 2-0 by Liverpool at Anfield on Saturday.
The midfielder hasn’t been a regular starter since recovering from a back injury late last year.
However, with Shelvey, out for two weeks at least, out of Saturday’s home game against Southampton with a knee injury, Merino could be set for a run in Rafa Benitez’s side.
The 21-year-old tried to be positive against Liverpool, but he tired in the second half.
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And Merino is hopeful he can rediscover his early-season form with a run of games.
“I felt tired out there,” said Merino, signed on a permanent deal from Borussia Dortmund early in the season.
“When you’re not playing regularly, that’s normal, because minutes on the pitch give you energy.
“I feel good. Of course, it’s not easy when you are defending all the game, but I worked hard and tried to do what the coach wanted.
“Jonjo Shelvey may be missing again, so I have to be ready for that too.
“I need to take advantage of an opportunity I get. I have recovered from my back injury, I have had other problems too, but I now feel fit. I’m training with high intensity and ready when the coach needs me.”
Newcastle have a 21-day break after the Southampton game, which is followed by a March 31 home fixture against Huddersfield Town.
Both teams are around relegation-threatened United in the Premier League table.
“Every game’s important, and we try to approach every one with the same mentality, but maybe the next two games at home are a little bit easier than this one,” said Merino.
“Not at the high level of Liverpool, but we know the way we have to play is the same as we played against Manchester United and Bournemouth.”
Newcastle kept free-scoring Liverpool out for 40 minutes.
However, their quality eventually told and Mohamed Salah opened the scoring after being played in by Alex Oxlade-Chamberlain.
Sadio Mane added a second goal in the 55th minute.
“Liverpool are a top side, a big club with quality players,” said Merino.
“They’re winning a lot of games while we are battling against relegation. There was a big difference between the two sets of players technically, but we tried out best, tried to defend together but in the end, they took their opportunities.
“It is difficult to defend for 90 minutes because they pass the ball so well and drag you out of position.
“They find spaces and you end up exhausted. Once they scored their first goal, it made it easier for them.
“In the second half, I think we kept pushing until the end and that is the positive we have to take from this game.”
“They’re similar (to Manchester City). Of course, they have now lost (Philippe) Coutinho, who was a very important player for them, but they still have so much going for them and they are showing that in every game.
“That is why they are winning most weeks.”
Newcastle avoided a heavy defeat at Anfield, and their goal difference is better than most of the teams in the bottom half of the table.
However, that was not much consolation to Merino when he left Anfield.
“Our goal difference is always important for us, and when we play big teams, we are not losing by big margins,” said Merino.
“Everyone can see the team is working hard, and we know goal difference will be important.
“Of course, it is not good to lose a game. We were not keeping it tight just to avoid conceding goals. We tried to keep it tight because that was the way we could get something out of the game.
“But no one wants to lose heavily. The less goals we concede, the better it will be for us.
“Some teams have come here and lost by many goals. I’m not saying there is a good way to lose.
“A loss is a loss and in one way it does not matter if you lose 1-0 or 4-0. It is still zero points.
“It is not about much better psychologically to lose narrowly, but it could be better position wise at the end of the season. Goal difference could be crucial.”
Merino says United simply have to get a result against Southampton ahead of their extended break.
“After that game, we have a three week break, and it’s always good to pick up a good result at a time like that,” said the Spain Under-21 international.
“If you have a defeat, you have a lot of time to dwell on it. It’s the worst thing to do, because you have so much time to think about it, but we have no intention of losing it.
“We want to go into the break with a good mentality.”