By Sedara Oluwadara   Manchester United are on the verge of completing their third summer signing after agreeing a £45m deal with Ajax for Argentina defender Lisandro Martinez.   United manager Erik ten Hag was keen to be reunited with the 24-year-old, who he signed for Ajax in 2019.       The Dutch club were reluctant to let Martinez leave but United chief executive Richard Arnold and football director John Murtough progressed the deal when they visited Amsterdam last week.   Martinez prefers to play on the left side of two central defenders and Ten Hag appeared to