Pep Guardiola is the most successful manager of the 21st century, with a mammoth tally of 31 titles across three of Europe's elite clubs. The 50-year old boasts a stellar resume that includes two Champions League titles, three Bundesliga and as many La Liga trophies, and three Premier League titles, among other domestic and continental honours.Guardiola scripted history in his four-year spell at Barcelona, leading the Blaugrana to an unprecedented sextuple of Champions League, La Liga, Copa Del Rey, FIFA Club World Cup, UEFA SuperCup and Supercopa de Espana in 2009. The former Barcelona player also went on to enjoy success in his subsequent two assignments at Bayern Munich and Manchester City.While the likes of Lionel Messi, Pedro and Sergio Busquets excelled under the Spaniard, a few others have struggled to make an impact under Pep Guardiola's tutelage. Without further ado, let's take a look at five such players.#5 Joe HartJoe HartManchester City legend Joe Hart was a crucial element in his team's 2011-12 Premier League glory, which was City's first league title since 1968.Hart's tally of 17 clean sheets that season earned him the Golden Glove award. He went on to win one more Premier League trophy before Pep Guardiola's arrival. The 34-year old's last trophy with City was the 2015-16 League Cup, after which Guardiola took over the reins from Manuel Pellegrini.After making 347 appearances in a decade at the club, Joe Hart managed to play just one solitary game under Guardiola. The former England No.1 was immediately relegated to the bench on the Spaniard's arrival, with Guardiola signing Claudio Bravo from Barcelona to play the role of a 'sweeper keeper'.🗣 | Pep Guardiola:“[That win was] for Pablo Zabaleta, Joe Hart, Vincent Kompany, many players that help us to be here."[via @MikeMinay] pic.twitter.com/rWOn3OhKIh— City Chief (@City_Chief) May 4, 2021During Pep Guardiola's first transfer window as Manchester City manager, there was a flurry of arrivals at the Etihad as most of Pellegrini's squad was cleared out.Joe Hart was sent out on a loan spell to Torino, followed by a move to West Ham, before he was permanently sold to Burnley after spending over 14 years at City. That dealt a massive blow to the Englishman's confidence, as he lost his place in the national side after his exile from the Etihad.#4 Benjamin MendyBenjamin MendyPep Guardiola's arrival at the helm of Manchester City led to a massive spending spree, with over £300 million spent on defenders alone.Benjamin Mendy was one of three full-backs signed by City in the 2017 summer transfer window. But the Frenchman was plagued by injuries, making only 23 appearances in his first two seasons at the club, as a knee ligament injury and subsequent surgery kept him out for the better part of two years.Following his return from injury, Mendy made 30 appearances in the 2019-20 season but failed to impress Pep Guardiola for a sustained run in the playing XI.Just dawned on me that at the end of the season Benjamin Mendy could have his third Premier League medal as well as being a World Cup and Champions League winner.The guy must have one of the best game-per-trophy ratio in the world 😂 pic.twitter.com/dvrGfnySHP— Footy Accumulators (@FootyAccums) May 5, 2021Oleksandr Zinchenko was signed in 2016 as a young prospect and was promoted to CIty's star-studded lineup after an impressive loan spell at PSV Eindhoven. Mendy has made just 20 appearances this season, as Zinchenko has effectively become the Cityzens' first-choice left-back under Pep Guardiola.