After being sidelined with a horror run of injuries, Calver played his first game for eight months against Western Sydney Wanderers last Saturday.
After what he a described as a "rusty start" he settled down well in central defence and handled the enormous pressure of a Sydney derby match.
Former Eredivisie player Buijs, who was signed last week, began training with the Sky Blues on Tuesday.
It remains to be seen how soon Sydney coach Graham Arnold will declare the Dutchman ready to play, but there is only one spot in the central defence up for grabs with Alex Wilkinson maintaining his stranglehold on the other.
Calver says he's up for the fight.
"I'm at that stage now where I need to be playing games regularly and to try and lock down a spot for as long as I can," he told The World Game.
"My contract is up at the end of the season. Obviously I'd like to stay at Sydney FC, but there haven't been any talks yet so I have to just wait and see. My manager will look after that.
"I hadn't really been training until recently, so I wasn't in a position to start talking about contracts. Get fit again first and then get back playing was the aim and hopefully I can move forward now and still be here at Sydney again next season.
"Top of the table and undefeated - who wouldn't want to stay here?"
Calver, who turned 21 on January 12, hyper-extended his left knee and also tore the lateral ligament and pulled the hamstring tendon off the bone while playing in an Asian Champions League game in May.
After being back in training for just two weeks he landed awkwardly at training and tore the meniscus in the same knee.
There had been a flurry of movements in the central defensive area at Sydney that opened up a spot for Calver against the Wanderers.
Matt Jurman went to Asia and Seb Ryall was injured, fast-tracking Calver's return after only a short base of full training.
Ryall will be back from a hamstring injury sometime later in the season and regardless of Buijs being here now Calver is hoping to get another run against Adelaide United at Allianz Stadium on Friday.
Then there is the latest "Big Blue" against Melbourne Victory at Etihad Stadium on Australia Day.
"Yeah, definitely, I'd love to get a run in those big games," he said.
"I think because we haven't lost a game we've got a big target on our back, so every team lifts to play against us and that's what we want, because it becomes a big challenge for us to play against them too.
"Every team wants to knock you off and each week the boys are really getting themselves up and rising to the occasion."