A player's market value is influenced by a variety of factors like age, ability and most importantly form. And some of these players have looked like a shadow of their former selves this season.Just a couple of years ago, footballers were getting transferred from one club to another for astronomical amounts. A raft of players were being valued at around €100 million. The pandemic has had a say on that in recent times.Regardless, when one is playing for the biggest clubs in Europe, they will usually have a high market value. But they will need to back it up with performances to match. We have seen many big-money transfers happen over the past few years and unfortunately, many of them have proved to be failures.What happens with flop transfers is that the buying club will incur losses if they subsequently sell the player they've bought because his transfer market value would have dipped. Without further ado, let's take a look at five stars whose market value has dipped the most in 2021.#5 Antoine Griezmann (Barcelona) - €60 million (€20 million dip)FC Barcelona v Athletic Club - Supercopa de Espana FinalOne touted as a direct rival to Cristiano Ronaldo and Lionel Messi during his heyday at Atletico Madrid, Antoine Griezmann has failed to kick on in the same vein. He is, by no means, a bad player. But he is just not the player Barcelona thought they were getting when they paid €120 million to sign him from the Rojiblancos.Griezmann's transfer market value has gone down from €80 million to €60 million. After having an underwhelming debut season at Barcelona, the Frenchman has improved. He has scored 16 goals this term across all competitions and has provided six assists as well.It's hard to fault Barcelona for thinking he'd produce numbers better than that. He has also turned thirty and his current market value is exactly half of what Barcelona paid to secure his services.Antoine Griezmann's first major trophy in Spanish football meant the world to him 👏📸 @AntoGriezmann pic.twitter.com/eJJ4cBXOv0— Goal (@goal) April 18, 2021#4 Philippe Coutinho (Barcelona) - €40 million (€20 million dip)FC Barcelona v Levante UD - La Liga SantanderPhilippe Coutinho will go down in history as one of the biggest transfer flops of all time. Barcelona paid close to €150 million to sign him from Liverpool. After impressing in his first half-season, Coutinho struggled to fit in at Barcelona and was shipped out to Bayern Munich.He enjoyed a great outing in the Bundesliga where he won the continental treble with the Bavarians. He started playing decently upon his return to the Camp Nou but has now been sidelined with an injury and it doesn't look like he will feature again this season.Coutinho's transfer market value has dipped from €60 million to €40 million this year and Barcelona might try to cut their losses and cash in on him this summer.Liverpool can say goodbye to another 20 million euros for Philippe Coutinho. The player will be out for a long time due to injury, and isn't likely to reach 100 games as a Barça player. He now has 90. [sport] pic.twitter.com/htf2WzIWVc— barcacentre (@barcacentre) January 1, 2021