Celtic started their 2009–10 campaign by playing in the Wembley Cup, as a buildup for the Champions League. Their first competitive game of the season was at Parkhead on 29 July where they were beaten 1–0 by Dynamo Moscow in the 1st leg of the Champions League third-qualifying round for best placed teams. However, in the return leg at Dynamo Stadium on 5 August, Celtic won 2–0 (2–1 on aggregate) thanks to a 44th-minute goal by Scott McDonald and a late injury-time strike from Georgios Samaras, giving Celtic a first away win in Europe since their 1–0 win over MTK Hungaria at Stadium Puskás Ferenc in 2003. The win took Celtic through to the Champions League play-off round and were drawn against Arsenal. Celtic kicked off their Scottish Premier League campaign with a 3–1 win over Aberdeen at Pittodrie Stadium.

After 30 games of the season, Tony Mowbray was sacked following Celtic's 4–0 loss to St Mirren. Neil Lennon was then appointed as caretaker manager until the end of the season; his first match in charge was a 3–1 victory over Kilmarnock on 27 March. After his first win, Lennon went on to win another seven games in a row, but was unable to lead Celtic to a win in the semi-final against Ross County, who beat Celtic 2–0. This was Lennon's only defeat during his tenure as interim manager. At the end of the season, Lennon was seen as favourite to succeed Tony Mowbray as the next Celtic manager on a permanent basis.

...
Wikipedia