On 12 August 2014, Celtic signed Denayer on a season-long loan from Manchester City. Denayer revealed that Vincent Kompany and Patrick Vieira played a vital role for him in joining Celtic. He was one of Ronny Deila’s first major signings as manager of Celtic.
He scored on his debut and was named man of the match in a 6–1 victory at home to Dundee United in the Scottish Premiership. Four days later, on 20 August 2014, Denayer made his Champions League debut, in a 1–1 draw against Maribor, where he was part of the starting line-up. His second goal came in a 2–1 win over Aberdeen on 13 September 2014. The defender scored again for Celtic on 18 October 2014, in a 5–0 win over Ross County. By the end of December, Denayer had made 17 appearances for Celtic and formed a solid partnership with Virgil van Dijk in central defence. Denayer commented about playing alongside Van Dijk: “We play well together and he is a very good player”, adding “I am also learning from him as well. He has more experience than me and he knows what to say to me. I listen to him because I know he can help make me a better player.”