Arsenal are third in the Premier League behind Tottenham Hotspur on goal difference and face potentially decisive fixtures over the next few weeks.
They take on Championship (second-tier) side Hull City in the FA Cup on Saturday followed by the first leg of their Champions League last-16 tie against Barcelona in midweek and a Premier League trip to Manchester United the weekend after.
"For Danny to go through a bad, injured period where he kept believing in himself, when everyone maybe doubted him, it was a great chance for him to come in and show he was ready to give everything for the team," Mertesacker told the club's website (www.arsenal.com).
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"We have long-term injuries, which gave us a bit of a struggle, especially in January, where we played in December and January with almost the same team.
"We are happy to have Danny now and a few others are not far away. We need our players -- you could see that makes a difference."
(Reporting by Simon Jennings in Bengaluru; editing by Sudipto Ganguly)
