It didn't start particularly well, but his performances improved as the season went on and he formed an important partnership with Koscienly in the second half of the season. The centre-back scored 3 important league goals and received his third career red card. He played the third most games in the premier league behind Cazorla and Arteta. Statistics are taken from whoscored.com.
Here are his statistics in the premier league:
The most interesting statistic is the fouls committed compared to times fouled. It is quite humorous that a defender, especially a centre-back, is fouled more than he is fouling. This is because Mertesacker is scared to tackle and would rather intercept/block/wait for the opponent to make a mistake. He had less successful tackles (1) per game than Podolski (1), Oxlade-Chamberlain (1.1), Cazorla (1.9) and Rosicky (1.2) among others. This characteristic is almost opposite to Vermaelen who rushes into tackles too often. Giving away so few fouls is good, but he should tackle a bit more; sometimes giving away a foul stops a goal.
Per won 2.6 aerial duels per game, second only to Olivier Giroud for Arsenal (3.9), with his immense height, he should win a few more than he did. Although he doesn't attempt many risky passes, he had the highest percentage of successful passes (tied with Arteta at 91.5%). He finished second worst out of the defenders in interceptions per game (1.5), only better than Sagna (1.3). As would be expected the 3 main centre-backs had the highest clearances and blocks per game: Koscielny (4.9)(0.4), Mertesacker (4.8)(0.6) and Vermaelen (4.5)(0.7).
The statistics definitely don't tell everything. Mertesacker was an important part of the team this year. If he and Koscielny continue their good partnership up we could be in for a trophy-winning season.
My overall rating: 7/10