He said: "If you sign a player with the purpose of covering for another player that generally means you rarely use them because they are not up to the required Premier League level. You have to be careful.
"This is the most difficult window to deal in and I keep saying this, it is a hugely flawed system. It stops me progressing the club, it stops me being more successful and it stops you at critical times overcoming injury problems in what can be crucial positions."
Allardyce added: "If you can find one in this country then they have Premier League experience so it is not going to be a mistake. The problems come if you go abroad. If it is from another country, it is difficult for them to come in and make an immediate impact because they have not played in English football before."