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Late in the contest at Stamford Bridge Mustafi rose to meet a free-kick and headed the ball past Thibaut Courtois.
The German defender wheeled off in celebration, unaware that the linesman had raised his flag.
It was the correct call from the official with Mustafi beyond the last Chelsea defender.
But Mustafi initially believed he was onside – revealing he mistook team-mate Kolasinac for a Chelsea defender.
‘I saw someone behind me, so I thought I wasn’t offside,’ he told Arsenal Player. ‘I knew I started a bit early, but I know as a defender it’s hard to stay in one line, so you get some going early, some a bit later. I thought if I start a split-second early, I might get this advantage.
‘I got it but unfortunately the man behind me was Sead and no one from the Chelsea team. I was really unhappy with that but it was offside.’
The goal would have broken the deadlock at Stamford Bridge and quite possibly led to Arsenal claiming three points instead of just one.
Despite failing to win the match, Mustafi took the positives from the match.
‘We are a team that are willing to learn. I think that we have to try and build from something and I think we can build form this performance today because we were really well organised when we didn’t have the ball.
‘When we had the ball, we played some brilliant one-touch football. I think we can build from this. We played today against the champions of last season and if you play like this, I think you’ll get a lot of points. We can build from that.’
Arsenal host Doncaster in the League Cup on Wednesday before playing West Brom at the Emirates in the Premier League.