Former mentor and apprentice will face off on the sideline at Old Trafford on Sunday, with Manchester United boss Luis van Gaal believing former student José Mourinho has already begun the battle of wits. Chelsea boss Mourinho worked as part of Van Gaal’s coaching staff for three years at Barcelona in the late 1990s before the Portuguese went onto join the Dutchman among the world’s elite coaches.

The pair met for the only time in opposition benches when Mourinho’s Inter Milan triumphed over Van Gaal’s Bayern Munich in the 2009 Champions League final. They now meet for the first time in England in what has the potential to be one of the great Premier League rivalries after Van Gaal took charge at Old Trafford in the summer. Adding further intrigue to the battle is that many believe Van Gaal now sits in the seat that Mourinho coveted following the retirement of Sir Alex Ferguson.

And, while the two are “big friends,” according to Mourinho, there will be no easing up from two of the sport’s fiercest competitors and most self-confident men, a fact evidenced by discussion over the fitness of Chelsea striker Diego Costa. The former Atletico Madrid man has made a flying start to his career at Stamford Bridge, notching up nine goals in seven Premier League matches. But a long-standing hamstring problem was aggravated while on international duty with Spain earlier this month and this week he spent a night in hospital with a virus.

“Diego has a chance, he has a little chance,” Mourinho said, according to Chelsea’s official website. “Everything happened to him. He was obviously injured and then he had to go to hospital before the Maribor game with a viral situation that we didn’t manage to resolve without him being in hospital overnight.

“We are trying and we will see what happens between today and tomorrow. I’m waiting until after tomorrow’s training session to see which players are available. I will consider nothing until then. Obviously I have studied Man United the best I could, as I always do with every opponent, but I haven’t made any decision about my team.”

Without Costa, Mourinho’s options up front would be few and far between. Loïc Rémy suffered an injury while scoring Chelsea’s opening goal in a 6-0 win over Maribor in midweek and is definitely ruled out. Didier Drogba replaced Rémy but the 26-year-old has played precious few minutes since returning to the club in the summer and has recently been troubled by his own injury. The only other realistic alternatives would be to deploy Andre Schürrle in a false nine role as Mourinho dis last season at Old Trafford, or even field Cesc Fabregas in that position. Van Gaal, though, hinted that he thinks Mourinho is deliberately underplaying Costa’s chances of playing.

“I think Costa will play,” Van Gaal said, reports The Telegraph. “He can play maybe because of what I have heard and what I have read on the websites that he has trained this week. I also have my eyes open!

“He has scored nine goals out of seven games, which is remarkable. He is in good shape but he was ill yesterday, so who knows?”

Van Gaal has left no such doubts about the fitness of the man who has undoubtedly been Manchester United’s standout player since his club-record transfer from Real Madrid, Ángel di María. The Argentine midfielder had to be substituted during his team’s 2-2 draw with West Brom on Monday but is now fit to join Spaniard Juan Mata in going up against his former manager. Michael Carrick, Antonio Valencia and Jonny Evans remain unavailable, while Wayne Rooney serves the final match of his suspension.

Prediction: At first glance this is a match that appears perfectly set up for a Chelsea win. Mourinho has made an art form of getting his teams to hang back before ruthlessly exploiting the opposition on the counter attack. And under Van Gaal Manchester United continue to look top-heavy and extremely vulnerable at the back.

But that to some extent ignores the potential for two of the smartest coaches around to come up with something unexpected. Will Mourinho focus on quieting United’s attacking threat and be content to leave Old Trafford with a draw, as he was away at many of the top clubs, including United, last season? And will Van Gaal come up with a way to mitigate his lack of defensive talent, as he did with so adeptly with the Netherlands at the World Cup?

A stalemate or even a narrow United win, in which they take advantage of Chelsea’s greater openness this season, certainly can’t be ruled out. But the balance of probability, based on the all-round strengths of the two teams, is on an away win for the Premier League leaders.

Kickoff time: Noon EDT

TV channel: NBC

Live stream: NBC Sports Live Extra