Swansea boss Paul Clement has expressed his surprise that Chelsea allowed Nemanja Matic to join Manchester United as he prepares for a reunion with the player.
Clement worked with Matic during his time as Carlo Ancelotti's assistant at Stamford Bridge before the holding midfielder joined Benfica in 2011.
Matic returned to Stamford Bridge two-and-a-half years later and was a driving force as Chelsea won two Premier League titles in three seasons.
But Chelsea sanctioned Matic's £40million transfer to Old Trafford last month - and the 29-year-old Serbia international made an impressive start to his United career with a man-of-the-match performance in the 4-0 season-opening victory over West Ham.
"I was surprised Chelsea let him go, I don't know how that deal managed to happen," head coach Clement said ahead of United's visit to the Liberty Stadium.
"It's not too good for Chelsea and Antonio Conte has made it clear he did not want that to happen. It is a win for United.
"He is good positionally, he has good physical stature, he is strong in the challenge and good in the air.
"He has also a nice left foot and is an under-rated passer and is very focused on his football."
Matic made only three senior appearances for Chelsea under Ancelotti, all as a substitute, before spending the 2010-11 season on loan at Dutch club Vitesse Arnhem.
"I was not sure about him when I worked with him, and I don't think Carlo was because he did not play much," Clement said.
"But at that time Chelsea had a really strong midfield - John Obi Mikel, Michael Essien, Frank Lampard and Michael Ballack.
"So it was always going to be hard for him to break in and the club decided to sell him to Benfica.
"But he did really well and they got him back in the end."
United have finished seventh, fourth, fifth and sixth since Sir Alex Ferguson stood down as manager in May 2013 after his 13th Premier League title triumph.
But Clement expects them to be challenging for the title this season after a summer of heavy investment.
Manager Jose Mourinho has added Matic, striker Romelu Lukaku and centre-back Victor Lindelof to a squad that now looks well equipped to compete on all fronts.
"They're going to be challenging and they look strong, particularly in the midfield and front positions," Clement said.
"They are becoming more of a Mourinho team, where there is athleticism and power and they are strong on the counter.
"They may not have the flair of some of the other more fluid passing teams, but they are very functional."
Swansea's own build-up to the game has been dominated by the Gylfi Sigurdsson transfer saga, which finally came to a conclusion on Wednesday when the Iceland playmaker joined Everton for a club-record fee around £45million.
Clement hopes to use those funds to sign two or three players before the transfer window closes on August 31.
But, for now, he has to work with the players who fought out a goalless draw at Southampton on the opening day.
"I am not sure what the expectation is of us in the game, but from my own point of view I want us to put in a good performance," Clement said.
"If we concentrate on doing that, the result will sort itself out.
"That is what we need to do, particularly after last week when we kept a clean sheet under pressure."