Negotiations are underway to bring the 22-year-old to the JJB Stadium in readiness for the club's Premiership debut.
Jewell met with Castro's representatives last Thursday and the Academica Coimbra defender is keen to move to English football.
The move could take some time to complete, though, with the Latics manager admitting discussions were "complicated".
Castro is highly rated and Academica, who play in the Portuguese Superliga, would like to keep hold of him.
Strengthen
But he's now in the final year of his contract and the club are tempted to cash in to fund new signings.
In an exclusive interview with the Evening Post, Jewell said: "I met his people in London last Thursday which wasn't the best of days considering the terrible events.
"At the moment discussions are ongoing.
"It's complicated but hopefully we can get it sorted out as he's a player we have been interested in for a while.
"He's a central defender and as everyone knows, that is one of the positions I want to strengthen before the start of the season."
Castro has been with Academica since he was a youngster and was a key member of their defence last season.
Sporting Lisbon and Celta Vigo have expressed an interest in him, as have West Ham.
There's a strong chance Castro was recommended to Jewell by Chelsea boss Jose Mourinho, as the managers have struck up a good relationship this summer ahead of their meeting at the JJB on August 14.
Jewell said: "I've talked with Jose a few times now and he's very supportive.
"A couple of weeks ago, he gave me a full rundown of players in Portugal, who he rated and recommended. He's very knowledgeable."
While Jewell attempts to do a deal for Castro, one player he won't be signing is Algerian midfielder Djamel Belmadi.
The former Marseille player, whose current club is Al Ittihad in Qatar, was linked with a move to Latics after Celtic rejected the chance to sign him following a trial.
But Jewell said: "We aren't signing him. He trained with us last Friday but that is all."
Jewell met with Castro's representatives last Thursday and the Academica Coimbra defender is keen to move to English football.
The move could take some time to complete, though, with the Latics manager admitting discussions were "complicated".
Castro is highly rated and Academica, who play in the Portuguese Superliga, would like to keep hold of him.
Strengthen
But he's now in the final year of his contract and the club are tempted to cash in to fund new signings.
In an exclusive interview with the Evening Post, Jewell said: "I met his people in London last Thursday which wasn't the best of days considering the terrible events.
"At the moment discussions are ongoing.
"It's complicated but hopefully we can get it sorted out as he's a player we have been interested in for a while.
"He's a central defender and as everyone knows, that is one of the positions I want to strengthen before the start of the season."
Castro has been with Academica since he was a youngster and was a key member of their defence last season.
Sporting Lisbon and Celta Vigo have expressed an interest in him, as have West Ham.
There's a strong chance Castro was recommended to Jewell by Chelsea boss Jose Mourinho, as the managers have struck up a good relationship this summer ahead of their meeting at the JJB on August 14.
Jewell said: "I've talked with Jose a few times now and he's very supportive.
"A couple of weeks ago, he gave me a full rundown of players in Portugal, who he rated and recommended. He's very knowledgeable."
While Jewell attempts to do a deal for Castro, one player he won't be signing is Algerian midfielder Djamel Belmadi.
The former Marseille player, whose current club is Al Ittihad in Qatar, was linked with a move to Latics after Celtic rejected the chance to sign him following a trial.
But Jewell said: "We aren't signing him. He trained with us last Friday but that is all."