David Moyes has admitted that Manchester United could fail to reach the Champions League knockout stages, as he turned his attention to Wednesday's trip to Ukraine to face Shakhtar Donetsk.

The United manager cited Manchester City's inability to do so last year, when they finished bottom of their group.

The pressure is starting to build on the new manager as he oversees a start to the club's title defence in the Premier League that has yielded only seven points from six games.

On Saturday United were handed a shock 2-1 defeat by West Bromwich Albion, the first time the Midlands club have won at Old Trafford since 1978.

United now travel to for Wednesday evening's second Champions League group game. Despite United having won their opening game 4-2 against Bayer Leverkusen a fortnight ago, Moyes believes the greater spending power of clubs such as Paris Saint-Germain and the continuing development of high-quality Spanish players make it ever more difficult to compete against the continental elite.

He said: "It has become a harder competition to win, with the spending power of some of the big teams in other countries. With the emergence of all the good Spanish players and Spanish teams and now you have PSG – the competition has become much more open than it was in years gone by."

United did not emerge from the group phase two years ago under Sir Alex Ferguson and Moyes made a clear warning that this could be repeated this year. "It's been hard for Manchester United as well and progress is not something that is guaranteed," he said. "There were no guarantees for Manchester City [who failed to reach the last 16] last year. You have to play well enough and grow into it and, hopefully, keep moving along."

Against West Brom Moyes rested Nemanja Vidic, Rafael da Silva, Marouane Fellaini, Patrice Evra and Robin van Persie from the starting XI, a decision that backfired with defeat. For Wednesday's trip to the Donbass Arena he is intent on fielding a competitive side. "I think we need to go there with a strong team," he said. "Going to Ukraine is tough because Donetsk have had a good record. I know they've sold one or two players but they have five or six Brazilians – they are not all Ukraine-based players but from all over the place.

"Bayer Leverkusen was fantastic and I'm really looking forward to Wednesday and seeing what it's like getting to be involved in these competitions. I think the history here is to be known throughout Europe; Manchester United and their history shows you that has been the case."Regarding United's shaky opening to the campaign, Moyes added: "I think we said at the start it could be a topsy-turvy Premier League season and it may well prove to be that. "

The Scot is glad of the chance to play Shakhtar to try and move on quickly. "Yes, they all come fast here," he said. "It's a real tough game but it's coming about and we'll move on. As I said, it's going to be a bit like this at times but we'll get it right in the end. I think the Manchester United players set really high standards. They are all top lads. But you're the manager, you take it, that's what happens and you know what the consequences are because if the team loses it's always the manager."

Despite stating after the 4-1 derby defeat that there would be more difficult days, Moyes was unhappy another came in the next league match. "I didn't expect one to come so soon but I think it's going to be the way," he said. "I'm taking over a new team, the team has got a new manager and I think there will be some setbacks along the way. But I think there are other clubs having setbacks as well, not just our club. The only one I have to concentrate on is mine, though, and I'm obviously disappointed with the performance [on Saturday].

"We wanted to try to kick on, that's what we wanted but we'll do it next time. That's the plan. If you don't do it, you have got to go again and we'll try and do it in the next game."