#2 Sevilla (9 goals conceded in 10 games; 0.9 conceded goals / game)

Sevilla came through three rounds of qualifying to top their 2018-19 Europa League group, and beat Lazio 3-0 on aggregate in the Round of 32. In the Round of 16, a 6-5 aggregate defeat to Slavia Praha ended Sevilla's quest for four consecutive Europa League titles in as many appearances in the competitions.
In 2019-20, Sevilla were the only team to win five games in the group-stage of the Europa League, with their lone European defeat of the season coming on Matchday 6 against Qarabag.
Sevilla's tally of 6 European clean sheets during the year is only bettered by Chelsea (7) and Barcelona (7) while the Andalusian club's match win rate of 70% (7 wins in 10 games) is equaled only by Manchester City (also 7 wins out of 10).
#1 Barcelona (9 goals conceded in 12 games; 0.75 conceded goals / game)

Following a goalless draw at Lyon in the first leg of their 2018-19 Champions League tie, Barcelona overwhelmed the French side 5-1 in the return leg before easing to a 4-0 aggregate win over Manchester United in the quarterfinals.
A Lionel Messi masterclass in the first leg of the semifinals against Liverpool gave the Liga side a commanding 3-0 aggregate advantage. But Barcelona's hopes of reaching their first Champions League final in 4 years came undone in a 0-4 reverse at Anfield in the return leg.
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The two-time defending Liga champions started their 2019-20 European season with a goalless draw at Borussia Dortmund. Club captain Messi became the first player to score in 15 consecutive seasons and also score against 34 different teams in the competition as Barcelona reached the Round of 16 as one of 6 unbeaten teams.
With 9 goals conceded in 12 European games in 2019, Barcelona have the lowest tally of goals conceded per game of any team to have played over 10 games during the year. Barcelona's 7 European clean sheets during the year is equaled only by Chelsea (7 clean sheets in 15 games).