Millwall have had a difficult start to the season, which has seen long-serving manager Neil Harris leave his job.

Former striker Harris had been in permanent charge since the end of the 2014-15 season, and led The Lions to promotion back to the Championship - via the play-offs - in 2017.

A highly impressive finish in the second tier followed for Harris' side, and after a tougher 2018-19 campaign, the 42-year-old still marshalled his to side to Championship football for another season, securing safety with two games to spare.

During his tenure, Harris was reluctant to over-utilise the loan market, especially when it came to would-be loans from Premier League clubs.

He felt that - despite acknowledging the increasing attractiveness of the market - due to the demands these clubs would make, such as game-time guarantees and loan fees, the risk-reward ratio often wasn't high enough,

Indeed, on the subject of Premier League loan signings, over the summer he told newsatden.co.uk: "I'm all for taking players and giving us the opportunity to strengthen us, but it has to be on our terms as well."

Of course, that is not to say the Essex-born manager hasn't used the loan market to his advantage at all.

There are numerous examples since 2015 where Harris has taken players on loan from other League clubs and have consequently become a big success at the club.

Here, we look at the three who have made the most significant positive impact:

Jake Cooper

Central defender Jake Cooper signed on loan from Reading in January 2017 on a deal until the end of the season.

Having made only three sub appearances for his parent club that season, the Bracknell-born defender arrived at the Den looking for more minutes on the pitch.

The England U20 international got his wish, going on to make 20 starts for The Lions and scoring 2 goals as the club achieved promotion to the Championship.

Cooper made such an impact he was signed on a permanent deal that summer, and still remains at the heart of Millwall defence this season.

He has won Manager's Young Player of the Year twice, has made 111 appearances, and scored 12 goals.

Jed Wallace

Winger Jed Wallace spent two spells on loan at The Den from Wolves - first for one month in January 2016, then re-joining 12 months later for the rest of the season.

Across the two spells, the England U19 international made 28 league appearances, scoring four goals and adding numerous assists as the South-east London side made two play-off finals, triumphing in the second.

Such was his positive impact, the club acted to sign the Reading-born man permanently at the end of his second spell, and is still an integral part of Millwall's squad.

In his first two seasons as a permanent squad member, he scored 11 goals and assisted 11 more from the wing, continuing to flourish under Harris.

Of course, his impact shows no sign of slowing and he has already bagged four goals this season.

George Saville

Northern Ireland international George Saville is another player who had two loan spells and proved to be a real success at the Den, even if he only truly excelled as a permanent player

The first - in 2013 - was as a Chelsea youngster and didn't prove particularly successful, ending after just three appearances.

His second spell - this time under Harris in 2015 - was more far more productive for both parties.

Saville played 12 games in three months for the South-east London side as they pushed for promotion to the Championship.

When the chance arose to sign the central midfielder in the summer of 2017 after promotion, his former manager jumped at the chance.

Indeed, speaking to the Millwall Official Website after completing Saville's permanent signing, Harris spoke of his delight:"I'm really pleased. George is well-known to us as he's been a player here before. He had a really successful loan with us and we've monitored him ever since."

By the time his first season had ended - with The Lions in eighth place - the Camberley-born player had fully justified his manager's faith, scoring 10 goals in 44 games.

Of course, this is a significant goal return for a midfielder, and it is unsurprising he made the WhoScored.com Team of the Season.

Unfortunately for Millwall fans, he signed for Middleborough at the start of last season for an undisclosed fee.