It's no secret that Manchester United will need reinforcements in the upcoming summer transfer window. The club has underperformed to a great extent in recent times and much of it has been down to poor performances from their footballers.While existing players can improve, new talents are always required to keep things fresh and challenge the existing players for their places. With several big names entering the upcoming summer window as free agents, the onus will be even more on the club to get it right.Manchester United have to solidify their midfield and defenseThe crux of the problem at Old Trafford lies in midfield and defense. They just haven't been good enough in recent years, with the defense making too many errors in crucial games. At times, the midfielders have also looked short of ideas while their defensive work rates have not been up to the mark.It suffices to say that Manchester United's prime target in the upcoming free-agent market will be to cover these holes. Here are some players they could bring in on a Bosman move:#5 Marcelo BrozovicMarcelo Brozovic has been a loyal performer for Inter Milan.Manchester United have a massive shortage of midfielders who can play in defensive roles without giving up on the offensive front. Both Fred and McTominay have been deployed in such positions, but neither has been able to step up to the task consistently.United may have a cheap solution to this problem if they manage to secure the signature of Inter Milan midfielder Marcelo Brozovic. The Croatian was one of the best bargain signings when he joined Inter from Croatian club Dinamo Zagreb. Brozovic is a tough-tackling midfielder, known for winning the ball and starting attacks.👑🐉 Perisnitch@snajathsThrowback to when Brozovic captained Inter for the first time, in the 2020 Milan derby.We were losing 0-2... then Brozo orchestrated the historic 4-2 comeback, with him scoring the first goal from outside the box. Man of the Match performance that night.3:41 AM · Jan 11, 20221103139Throwback to when Brozovic captained Inter for the first time, in the 2020 Milan derby.We were losing 0-2... then Brozo orchestrated the historic 4-2 comeback, with him scoring the first goal from outside the box. Man of the Match performance that night. https://t.co/mtmCAQAojOHis current contract with the Nerazzurri expires at the end of the 2021-22 season, but that has not affected his performances. On the contrary, he has been a consistent member of the starting XI as Inter Milan look to defend their Serie A title.Brozovic has already made 28 appearances, and his primary duty has been to shield the defense. But he has chipped in offensively, contributing a goal and two assists.#4 Boubacar KamaraBoubacar Kamara (right) is one of the best talents in the Ligue 1 right now.Boubacar Kamara is one of the many talented youngsters currently coming up through Ligue 1. Playing for Marseille, Kamara grabbed the headlines with his brilliant displays for the French outfit at just 22 years of age.He began his journey as a centre-back before being shifted into his current defensive midfield role. His versatility is a prime reason United could look to bring him to Old Trafford. Additionally, Kamara is technically sound and doesn't rush into rash tackles.West Ham Transfers@westhamtransferWest Ham could sign Boubacar Kamara from Marseille this window for a bargain of £5 million, according to reports.2:58 AM · Jan 19, 202252411West Ham could sign Boubacar Kamara from Marseille this window for a bargain of £5 million, according to reports. https://t.co/U4bFQH5OiYKamara has already played plenty this season, though not always as a starter. He has made 23 appearances and has been reliable when called upon. Manchester United could potentially get themselves an incredible bargain by picking him up as a free agent in the summer.