The issue with Clarke is pretty similar to the issue we had with Foyth: they were both streaky players on arrival, and the only way to fix that is to have them play enough football that the poor decisions start to get ironed out and their instincts sharpen upThe loan move(s) did us and him no favours but in hindsight, we signed him at a time when we didn't have the luxury to wait around while young players developed. Too many senior players needed replacing immediately and to matters even harder, very few of them played in his preferred position so he was never likely to get game time.
Ironically signing him instead of Bryan Gil might have been a good shout in terms of timing but now we see the latter has gone through a similar trajectory due to the state of the first team itself i.e. we still aren't in a healthy enough condition to indulge young players while they learn the ropes here or elsewhere and for the same reason we aren't good enough in the cup competitions to at least give them solid game time (although Conte was philosophically against the concept in its entirety).
It's a huge jump up to the PL but if Brentford are after him, there is obviously confidence he can make it.
He was signed by a manager who probably assumed he'd be given years to build a second squad, but was only given 3 months.
In the case of Foyth, realistically we should have been braver in using him when he was available during the 2018-19 season, because while we were overloaded with RBs that season we really should have cut our losses on Aurier and have Trippier/Foyth as our rotation axis with KWP in reserve, because while injury did eat into his playing time, that would have set up the asymmetrical FB system Poch wound up introducing the following season (plus KWP would have been in line to get some playing time, due to offloading Aurier)
On the other hand, in Clarke's case the main issue was that Poch tended to like a pacey winger off the bench (which is what N'Jie and Nkoudou were) so in theory Clarke could have been an option the season he arrived, but due to him being such a raw talent when he arrived even if he did get subbed on for let's say 25% of our matches with a few cup starts thrown in, even if he remained injury free he likely wouldn't have started to truly click until either very late in 2019-20 or more likely at some point the following season
If things worked out and/or were handled differently, hindsight suggests that Foyth would likely be a starter for us by this point (with the likely knock-on effect of us selling Sanchez a couple of years ago) but Clarke is harder to gauge - partly due to his Championship loans seeming to be cursed, but also he'd have likely found himself in the same position Bryan Gil is currently in where there's a Sonny-shaped blockade to him getting regular minutes - although given he has played LWB for Sunderland this season, it does have to be asked if having Perisic/Clarke as our LWB axis this season was something we could have considered instead of letting him go last summer

