Sigurdsson, who had previously enjoyed a successful loan spell at the Liberty Stadium, joined from Tottenham on Wednesday, with Michel Vorm and Ben Davies moving in the opposite direction, while Ecuador international Montero came in from Mexican outfit Morelia the following day.
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"Gylfi was a player I wanted to bring in because of his quality, the goals he scores and the fact he has proven himself at this level," Monk told the club's official website.
"In the few days we’ve had him we can see his quality already. The group has noticed that, and it's had a positive effect.
"And the same goes for Jefferson. He's settled in very quickly, and I've seen a good reaction from him and the squad in training.
"Gylfi was an important signing for us. We have been tracking him all summer. The quality he has will raise the quality of the squad. He knows the club, the fans, the area and what is expected of him.
"Gylfi is a great learner. He is quick to pick things up and that’s part of why I brought him in. He can score goals, create chances and can play in a number of positions too.
"We have watched Jefferson in depth and a lot of work went on behind the scenes to make the deal happen.
"He has a lot of quality and with the attributes he has he adds another dimension to our game. The ideal situation for me is that I have different types of players in the same position because in a game I may need to use a different approach.
"Jefferson certainly offers us something different, and I'm really happy he is here."