The 25-year-old Inter Milan midfielder, a Euro 2016 winner with Portugal, has agreed a loan deal with the Hammers until the end of the season.

And with West Ham decimated by injuries and still firmly in the relegation mix, Moyes desperately needs Mario to adapt quickly to the demands of the Premier League.

"My biggest concern is not him as a footballer but getting used to the Premier League, the speed and competitiveness compared to Italy or Portugal,'' said the Hammers boss.

"His ability as a footballer is not in question, he's proved it, and I just hope he can hit the ground running. That's my only concern.''

Moyes urgently needed reinforcements to add to his increasingly threadbare squad, with Marko Arnautovic and Manuel Lanzini the latest players to suffer injuries.

The in-form attacking duo both have hamstring problems with Arnautovic facing at least a couple of weeks out and Lanzini maybe six weeks on the sidelines.

Andy Carroll is out for three months while Michail Antonio and Diafra Sakho are still short of fitness.

That leaves Javier Hernandez and Andre Ayew -- both have been linked with moves away from the London Stadium this month -- as Moyes' only senior forwards.

West Ham have received an offer for Ayew, who is a reported target for his former club Swansea, but Moyes revealed the bid will not be accepted "as it is''.

Newcastle midfielder Jonjo Shelvey has been rumoured to be on Moyes' wanted list all January but the Scot dismissed that link, and admitted he fears West Ham's business may now be complete.

"That was never in place,'' he said. "It was never talked about, never mooted.

"We didn't speak to Newcastle and Newcastle didn't speak to us.

"I would like to have a few more but the truth is I wouldn't want to say because I couldn't be confident we will get any more.

"At the moment I can't say there is any guarantee there will be anybody else coming.

"It's really difficult. Behind the scenes we have tried for two or three players and most of it is getting knocked back.''