3. Juan Mata
Arsenal should have ideally had no problems in signing the then 23 year old Juan Mata from Valencia in order to replace Cesc Fabregas. With the release clause of around £18m, he was always affordable. But with Chelsea also interested in Mata, his price was sure to rise due to competition. Due to some reason or another, Wenger did not pursue Mata with better determination, as Chelsea easily outbid the Gunners to a £27 million deal.
Mata went on to be worth every penny for the Blues as he became their player of the year twice in his two full seasons with the Blues. Mata was then seen surplus to requirements by new manager Jose Mourinho and was sold to Manchester United in the Red Devils’ record breaking £37.1m deal, highlighting the growth shown by the Spaniard.