New Blues boss Enzo Maresca has found himself under the spotlight following defeat by Manchester City in their Premier League opener, with Raheem Sterling and Ben Chilwell among the players told they were no longer in his first-team plans.
Following a lacklustre first-half, which saw a much-changed Chelsea team booed off by some of the disgruntled home fans, Christopher Nkunku broke the deadlock from the penalty spot.
Substitute Noni Madueke added a second with 15 minutes left to give Chelsea boss Maresca some welcome breathing space ahead of the second leg in Switzerland.
In the Europa League, Hearts slipped to a 1-0 defeat at Czech side Viktoria Plzen after an own goal from Daniel Oyegoke deep into stoppage-time.
Shamrock Rovers' hopes of making it to the group stages appear all but over after a 4-0 defeat away to PAOK in Thessaloniki.
The Dublin side also had Joshua Honohan sent off in the second half, but regardless of the outcome of the second leg at the Tallaght Stadium, Rovers are already assured of a spot in the Europa Conference League group stage.
Former Middlesbrough striker Chuba Akpom scored a hat-trick as Dutch giants Ajax ran out comfortable 4-1 winners away to Jagiellonia in Poland.
Elsewhere in the Conference League, Scottish Premiership side Kilmarnock lost 2-0 at Copenhagen, conceding what could be a costly second goal in stoppage-time.
The New Saints took a 3-0 advantage from the away leg against Lithuanian side FK Panevezys in Vilnius as they aim to become the first domestic Welsh club to reach the group stages of a European competition.
Danny Davies' header put the Cymru Premier champions in front early in the second half, with a well-taken goal from Dan Williams doubling their lead before substitute Ben Clark struck a third in stoppage-time.
Irish Premiership champions Larne were beaten 2-1 by Lincoln Red Imps of Gibraltar in Faro.
Matty Lusty put Larne ahead with a long-range effort after 16 minutes, but Kike Gomez soon had the hosts level with Spanish midfielder Joe completing the turnaround shortly before half-time when he slotted the ball home following a free-kick.
St Patrick's Athletic battled to a goalless draw with Turkish side Basaksehir at the Tallaght Stadium.
Fiorentina - runners-up in the last two Conference League finals - were held to a 3-3 draw at home by Puskas Akademia of Hungary.
Former Manchester United goalkeeper David De Gea had conceded twice in the opening 12 minutes on his first start before the Viola turned things around, only for the visitors to then score an 89th-minute equaliser.
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