Borja arrived from Atletico Madrid here on Thursday.
Guidolin believes that the 23-year-old, who scored 18 La Liga goals while on loan at Spanish football club SD Eibar last season, will prove to be a good value signing.
"He is a young player but I think he is a very good player. The future is in his hands. He is here to help us improve week after week, and for him to have the possibility to play," he said.
"I am happy to have him here as one of our players, and I am also happy with the group we have. I think Borja can justify his price tag. He was expensive, but I think he is a good signing. I think it's a good deal for us," he added.
Guidolin indicated that Borja will not be involved in Swansea's EPL opener against Burnley at Gawthorpe Hall on Saturday.
But Borja's fellow new signing, 31-year-old striker Fernando Llorente is expected to feature in the club's season opener as Swansea go in search of victory against Sean Dyche's team.
"Borja has just arrived and I have to respect the group, the players who did the pre-season in the USA from the beginning of July," Guidolin said.
Swansea have this week only sold Ashley Williams to Everton, and want a new centre-back to replace the departed club captain Willians as his last signing of the summer.
"We need a new central defender, although I am happy with my group and there is no rush," Guidolin said.
"We almost have two teams, with two players in each position, and that's good for me, but I am speaking with the chairman (Huw Jenkins) about a new centre-back. We have shortlists, but there's no rush and right now the focus is on our first game against Burnley.
"For me we only need one more signing -- a central defender," the Swansea manager specified.
Borja described his move to the club as the most important decision of his career.
"There is pressure, but it is good pressure, I want to repay the transfer fee paid for me with good performances and results," he said.
"It's difficult to set a target of goals, but my mantra is take it goal by goal. Hopefully I can score a lot of goals. It is the most important decision of my career. I want to settle and spend several years here," he added.
"I am very happy to be here. I am eager to contribute to the success of the team."
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