Over the last two transfer windows, Celtic boss Neil Lennon has helplessly witnessed his side's key players swap Parkhead for the Premier League. In the summer, he lost Victor Wanyama and Gary Hooper to Southampton and Norwich City respectively, while during January, Crystal Palace took advantage of Joe Ledley's contract situation to lure the midfielder to South London for a rather insignificant fee, believed to be around £800,000.

But this summer, it could be the Northern Irishman's turn to leave his Scottish Premiership days behind him in favour of its English equivalent. The Canaries, amongst others, have come calling at various points throughout the current campaign, and following three seasons of repetitive success with the Bhoys, one has to ask whether Lennon has achieved all he can with the Glasgow side.

When the 42-year-old took the Celtic gig back in 2010, it was undoubtedly an exciting challenge, the kind of post that balanced out great ambition with room for error, being the first job of the former midfielder's post-playing career. The eternal Glasgow-based battle for silverware was in full swing, the Old Firm derbies televised globally every season, and of course, the divine reward was experience in the Champions League, a must for any managerial CV, creating some unforgettable European nights in the process.

But Rangers' financial implosion and subsequent exit from the Scottish Football League drastically changed the landscape of the top flight. Unsurprisingly, in the absence of their only genuine title rivals, Celtic have walked away with the Scottish crown for the past two seasons, three including the last SPL campaign Rangers were involved in.

The current term has been most telling of the SPL's limited competitiveness; the Hoops have claimed 89 points out of a possible 102 with a goal difference of +66, enduring just one league defeat all season, whilst second-place Aberdeen lay some way off with 64 points - hardly what you'd describe as a genuine threat to Celtic's dominance. Along the way, goalkeeper Fraser Forster set a record of eleven consecutive clean sheets.

When the Parkhead side claimed the title again this year, the third consecutive one of Lennon's dugout tenure, the triumph was met with a pitch invasion, but in truth, the celebrations could have started as early as August. Celtic's title fate was already wrapped up before a ball was struck this season.

The Bhoys boss has taken the lion's share of the credit for his ability to meet expectations in the league despite the absence of competitive motivation and rightly so, yet the feeling that any manager of adequate credentials could record the same feats under the circumstances -  three titles, two Scottish Cups and two SFWA Manager of the Year awards - must be overwhelming.

In the interests of fairness, it's worth pointing out that without the smooth ride domestically, Lennon probably wouldn't have had the freedom to mastermind Celtic's noteworthy triumphs in the Champions League, with by far the most significant being his side's 2-1 victory over Barcelona at the Nou Camp in November 2012.

The allure of top European football has undoubtedly kept Lennon keen on the Celtic post. It's a competition only four Premier League managers are privy to per season, and amid the continual glut of the SPL, it's become the Craigavon man's only real barometer of success, where he can boast drastic overachievement in terms of finance, resources and the quality of player available to him.

But having plummeted out of the CL's group stages this season, with the squad intrinsically weaker than the year previous, it appears that even Celtic's European aims are beginning to saturate. Frankly, they can't compete with the continental elite, and even if the Bhoys were to drop into the Europa League next season, the chances of silverware remain particularly unlikely, considering its most recent winners have included Porto, Atletico Madrid and Chelsea - two of which are now facing each other in this season's Champions League semi-final.

Due to Lennon's ability to source bargain-bought starlets and develop them into sought-after talents in the space of a few seasons, in addition to Celtic's regular attendance in the Champions League proper, a study revealed earlier this year that the Scottish champions were amongst the top 40 earners in world football, generating £75.8million in revenues from the 2012-13 campaign.

That in itself is a sensational testament to what Lennon has achieved with finite resources at Parkhead, but impressive financial overheads aren't what the history books will eventually judge you upon. Likewise, the retired midfielder will be well aware that the January study into football finances also revealed that the Bhoys will never venture into the world's top 20 earners without a significant increase in revenues from SPL TV rights, suggesting Celtic's current trend of feeding the Premier League whilst defying the odds in Europe is unlikely to change for the better any time soon.

Perhaps I've been rather dismissive of Celtic in this article, but what is there left for Lennon to achieve at Parkhead? Another domestic double? Another upset in Europe? Another Manager of the Year award? Barring a treble and European silverware, or the rather tedious task of ensuring an undefeated league campaign, something Celtic are more than capable of doing, the Northern Irishman has already done it all. Meanwhile, his transfer budget continues to slim, as does the calibre of player available to him, whilst the added incentive of the Old Firm derby and the much-needed competition it would provide in the Scottish Premiership is still at least two years away.

Not that trophies would be on tap in the Premier League. In fact, many have speculated that Lennon would have to undergo a stint in the Championship first in order to prove his top flight credentials - a glaring insinuation of the lack of adequate challenge at Celtic Park.

But the Bhoys boss has reached his maxim with the Hoops and it can only go downhill from here. I'm not anticipating Celtic's spectacular fall from grace, but to paraphrase Charlton Athletic's Alan Curbishley, it's better to be clapped out of the front door before you're booted out the back.

This summer gives Lennon and the club the opportunity to part company on a high, the Hoops enjoying another title and the Northern Irishman leaving for a higher calling on good terms, having achieved all he can with the Scottish Champions. Whether his five trophies in three seasons is enough to secure a Premier League appointment however, remains to be seen.

[ad_pod id='ricco' align='center']