With no option to buy included in the loan agreement, the Croatian will likely stay in London for just one season. Kovacic is hoping to establish his credentials as a starting-quality midfielder for Madrid, which means he’ll have to impress during his time with the Blues.
If all goes well, the 24-year-old could prove to be one of the better loan signings in Premier League history.
Here’s a look at five of the best.
Carlos Tevez, West Ham to Manchester United (2007 to 2009)
After saving West Ham from relegation by scoring in the final game of the 2006/07 season against Manchester United at Old Trafford, Tevez joined his opponents on the day that summer in what was actually a two-year loan signing.
That two-season spell saw United win the Premier League twice, and reach the Champions League final both seasons, winning in 2008.
The Argentine scored 34 goals across all competitions, forming a lethal partnership with Wayne Rooney.
United decided against buying him outright at the end of his loan spell, and he made a controversial crosstown move to Manchester City, where he would win the league for a third time in 2012.
George Weah, AC Milan to Chelsea (2000)
Weah was past his prime by the time he joined Chelsea, at the age of 33, but he still made an instant impact in England after arriving in the second half of the 1999/2000 season.
Just by virtue of his reputation, his signing created a buzz, and though he only scored five goals for the Blues, they were all vital contributions.
He scored a goal apiece in FA Cup ties against Leicester City and Gillingham, firing Chelsea to the final, where they would go on to lift the trophy.
Weah then moved to Manchester City, where he was unable to scale the same heights of his earlier career, but he’ll be remembered fondly at Stamford Bridge.
Henrik Larsson, Helsingborgs to Manchester United (2007)
On the face of it, just three goals – one each in the league, FA Cup, and Champions League – seems like a middling spell, at best.
But signing Larsson after he left Barcelona was somewhat of a coup for Manchester United. The Red Devils’ players benefitted from the veteran’s presence, and he was such a natural fit in attack that Sir Alex Ferguson tried to extend his loan when it ended in March.
To this day, the United manager believes the rest of that campaign – which saw a league title, but a disappointing Champions League exit in the semi-finals against AC Milan – would have gone differently had Larsson been convinced to stay, and the Swedish legend has since admitted regretting his decision not to.
Jurgen Klinsmann, Sampdoria to Tottenham (1998)
Klinsmann was already a cult hero at White Hart Lane thanks to his first spell at Tottenham, when he scored 30 goals across competitions in 1994/95.
But what cemented his status as a Spurs icon was his second spell, when he arrived for the second half of the 1997/98 season. Tottenham were in danger of getting relegated when the German striker returned, but Klinsmann made sure there was no chance of that.
Nine goals in 15 outings fired the north Londoners to safety, and made him a darling of the White Hart Lane terraces forever.
Romelu Lukaku, Chelsea to West Brom (2012-13)
Romelu Lukaku had struggled to establish himself after joining Chelsea in 2011, but the next season, the club decided to give him playing time by sending him to West Brom.
The Belgian was an instant hit, scoring 17 times in 35 Premier League games for the Baggies. He was technically Chelsea’s highest league scorer that season, because none of the players who actually turned out for the club managed to top that.
His goals led West Brom to an eighth-place finish, their highest ever in the Premier League, and included a famous hat-trick against current employers Manchester United – in Alex Ferguson’s final game, no less.
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