Paul Merson has warned that Michael Carrick cannot afford to drop Bruno Fernandes and has challenged Matheus Cunha to deliver consistent performances, even as he praised Manchester United's dramatic 3-2 Champions League comeback against Roma on Tuesday.

United had fallen two goals behind before mounting a memorable recovery, ending a ten-game winless streak. Merson, however, insists the result should not mask the deep-rooted issues that persist at Old Trafford.

Bruno must be untouchable

"Bruno Fernandes has to play for Manchester United," Merson said. "He's their best player, and when you're going through a difficult period, you need your best players on the pitch."

Fernandes was notably left out for the Roma first leg, but Merson believes that cannot become a habit. "Even if he's not playing particularly well, he still gets into Manchester United's team. They haven't got a player better than him," he added.

Merson also referenced the legendary Brian Clough's philosophy: when times are hard, you pick your best eleven and stick with them. "Bruno has to be in it. As long as he plays well, they will always have a chance of winning."

The former Arsenal and England forward did note, though, that United must become a full 90-minute side. "They were very good against Chelsea in the first half on Saturday, and they had good spells against Roma, but you aren't going to win too many football matches playing like that. You need 70 or 75 minutes of good football, not 10 minutes here and 15 minutes there."

Cunha must find his level

Merson also turned his attention to Matheus Cunha, arguing the Brazilian's volatility is ill-suited to a club of Manchester United's stature. "He's a nine or a four. At Wolves, he could be a nine one week and they would win. The following week he might be a four and they wouldn't win. That's the nature of being at a club fighting towards the bottom of the table."

"But this is Manchester United. When you play for a club of that size, you've got to be a seven or a seven-and-a-half every week. These teams need to win week in, week out. They can't afford to rely on one outstanding performance every few weeks."

Merson also highlighted the adjustment facing Carlos Baleba after his hamstring injury, noting the stark contrast between playing for Brighton and playing under the relentless scrutiny of Manchester United.

Roma win showed character — and fortune

Merson credited United for the spirit shown in recovering from 2-0 down, but pointed out that fortune played its part. Fernandes earned the chance to retake his missed penalty, Roma gifted a goal through a defensive error, and the winning goal was one Merson felt a Premier League goalkeeper should have saved at the near post.

"It was a brilliant comeback and it could be the result that turns their season around," he said. "But I don't think you can look at that performance and say everything has suddenly changed. I feel sorry for Michael Carrick — I don't think it's him; I think it's the players."

Villa will provide a reality check on Sunday

Looking ahead to Sunday's Premier League meeting with Aston Villa at Old Trafford — kick-off at 4.30pm — Merson was blunt about United's chances. "I don't see them getting anything from Aston Villa on Sunday," he said. "Villa weren't great against Barcelona, but they're playing a much better standard of football at the moment. Roma covered over the cracks. I'm more interested in what happens this weekend from Manchester United."