THE irony in the transfer policy adopted by Celtic in recent seasons is that, to enjoy sustained success, they must regularly sell their best players.

The departure of Victor Wanyama to Southampton yesterday leaves manager Neil Lennon with a sizeable hole in midfield to fill but is further proof that the system is working.

Acknowledging their position as a big club in a relatively small market, Celtic have long given up trying to compete financially with clubs in England and in the bigger European markets when it comes to trying to attract established players.

Instead they have looked to source promising, untapped talent, often from underdeveloped markets, at a much cheaper price, with the hope of developing them, integrating them into the first team and, ultimately, selling them on for a substantial profit. It places a heavy responsibility on the club's scouting system – you have to have eyes and ears everywhere if you are to get to a hot property before an English Premier League club – but can prove highly lucrative if it all comes together as planned.

Ki Sung-Yueng was the first real example of the policy paying off – the Korean arrived for £2m in 2009 and was sold to Swansea City for a £4m profit last summer – and the margin is even greater in Wanyama's case. The Kenyan was signed for around £900,000 from Beerschot in Belgium in the summer of 2011 and has now agreed a four-year contract at St Mary's, moving for a fee thought to be in the region of £12.5m. Even taking into consideration a sell-on fee of around £600,000 to the newco founded by Beerschot after they became insolvent, it still represents a profit of around £11m from an investment made only two years ago. Lennon, naturally, would prefer to hold on to his best players, and Wanyama certainly falls into that category, but is also pragmatic enough to appreciate that a deal like this for a club in Celtic's position is simply too good to turn down.

The question now is whether and how Celtic reinvest that £11m income. Since Lennon became permanent manager in the summer of 2010, the club have only once spent more than £3m in a single transfer fee, shelling out in the region of £4m for Efrain Juarez. Alongside the £2.2m paid for Mo Bangura and the £1.4m for Daryl Murphy, it proved to be a costly, if rare, mistake.

Since then there has been a move towards securing players either out of contract or for knockdown prices. Of those still in the Celtic squad likely to be attractive to suitors with big pockets, Adam Matthews, Kelvin Wilson, Joe Ledley and Mikael Lustig arrived on free transfers, £100,000 was paid for Tony Watt, £300,000 for Kris Commons, £2m for Fraser Forster, and around £2.4m for Gary Hooper. If moved on, all would likely return a sizeable profit.

Last season's run to the last 16 of the Champions League has further bolstered Celtic's financial footing, but it is difficult to imagine Lennon now spending more than around £3m on any one player. So far this summer Celtic have paid £1.8m for Amido Balde and £2.6m for Virgil van Dijk, both just 22 and with potential sell-on value. The ideal replacement for Wanyama, therefore, would be sourced for a fee in that region (assuming the best free agents have already been snapped up this summer), possibly from an emerging market or, like Balde and van Dijk, from a lesser club in a more established country.

Replacing what Wanyama brought to the team, however, will not be that easy. The Kenyan looked most comfortable in a deep-lying holding midfielder role but had the physique and mobility to drive forward if space opened up in front of him. He further demonstrated his versatility via his occasional appearances in central defence.

Having just turned 22, his positional play and concentration levels, which were occasionally suspect, should only improve with age. With very few players of that calibre available at a reasonable rate in British football, Celtic will need to look to Europe or beyond to source something approaching a like-for-like replacement.

Of course, Lennon may instead look in-house. The arrival of van Dijk from Groningen as Wilson's likely central defensive partner next season would seem to free Efe Ambrose to move forward to fill the role of his departed friend. Ambrose, like Wanyama, has the power and pace to adequately step out of defence, and his occasional erratic performances – most notably against Juventus in last season's Champions League – may not prove so costly if he is playing higher up the park.

Then there is Biram Kayal. The Israeli has lost his way somewhat in the past 18 months or so after suffering ankle problems, and, by Lennon's own admission, has been a shadow of the player who lit up Scottish football upon his arrival. Kayal does not possess the power or strength of Wanyama, and sometimes shows a volatile temperament, but his tenacity and mobility mean he would seem the player most likely to replace Wanyama, in the short term at least. Regardless of who gets the role, those are big boots to fill.