before being replaced by Sergio Fernandez, in Cerro Porteño's 2–0 away defeat. In the return leg on 27 May, Caballero would play 69 minutes of the match before being replaced by Oscar Ferreira, as Cerro Porteño would win 3–0 and qualify for the semi-finals with a 3–2 aggregate score line. At the semi-final stage, Cerro Porteño faced Ecuador's Barcelona Sporting Club. After a 1–0 away defeat on 16 July, Cerro Porteño hosted the return leg at the Estadio Defensores del Chaco on 22 July. Caballero scored in the 4th minute of the match to level the scores 1–1 on aggregate, however, the match ultimately concluded in a 2–1 for Cerro Porteño, being 2–2 on aggregate. Barcelona went on to win via penalties, with Barcelona goal keeper José Cevallos saving Caballero's and two other of Cerro Porteño's penalties, eliminating Cerro Porteño from the competition. Caballero had scored 1 goal in 10 Copa Libertadores appearances. Loan to Arsenal On 29 October 1998, Caballero was loaned to Arsenal from Cerro Porteño for the remainder of the 1998–99 FA Premier League season (with an option for a permanent transfer), where he was issued the number #26. Caballero was also part of the club's reserve-team, playing in the 1998–99 FA Premier Reserve League and scoring 8 goals in 16 league appearances as the squad finished in 9th position of the table with 45 points. He played in the reserve-team alongside Nwankwo Kanu, Alberto Méndez, Jason Crowe and Kaba Diawara. His first goal for the reserve-team came in a 4–1 away defeat against Wimbledon on 4 November. Caballero would go on to score in the next three matches in a 3–3 home draw with Fulham (16 November), a 2–1 away victory against AFC Bournemouth (24 November) and a 2–0 home victory against Queens Park Rangers (30 November), totaling to a run of four goals in four consecutive matches. He made his debut for Arsenal's first-team as a substitute in a 5–0 home defeat by Chelsea on 11 November 1998 in the League Cup, entering the field for Dennis Bergkamp in the second half. On 29 November 1998, Caballero debuted in the Premier League in a 1–1 home draw against Middlesbrough, when he was substituted onto the field for Freddie Ljungberg in the 74th minute of the match. Caballero went on to make one more appearance for Arsenal, in a 4–2 FA Cup third-round victory away against