FC Seoul have been cruising this K League Classic football season thanks to their formidable offense, but they are now forced to make a change as skillful midfielder Sin Jin-ho bids adieu to the club for his mandatory military service.

Sin, who joined Seoul this season from the Pohang Steelers in the top-flight competition, entered the military training center in Nonsan, South Chungcheong Province, on Monday to begin his service.

The 27-year-old has been one of the instrumental players for Seoul's impressive run in both the K League Classic and the Asian Football Confederation Champions League.

In the domestic league, FC Seoul are in the first place with five wins and a loss, leading defending champions Jeonbuk Hyundai Motors by three points. In the AFC Champions League, Seoul are leaders in Group F with three wins and a draw. Seoul, the reigning champions of the Korean Football Association Cup, have poured in 28 goals in 10 matches. Sin has played in all of them.

While Brazilian striker Carlos Adriano has been the top contributor for Seoul with 14 goals -- five in the domestic league and nine at the ACL ---  Sin has been the lubricant of the team's attack strategy with accurate passes, creating scoring chances for Adriano and Montenegrin forward Dejan Damjanovic.

Though he played with Seoul for only three months, Sin showed no problems establishing chemistry with his new teammates. While taking free kicks and corner kicks for Seoul, Sin first made an impact by collecting two assists in Seoul's 4-1 victory over Japanese champions Sanfrecce Hiroshima at the ACL last month.

For his final match with Seoul before his military service, Sin notched a goal with a curling free kick and set up Damjanovic's goal, helping his club's 3-0 victory against Suwon FC on Saturday.

"Because I only had three months to play with Seoul, I wanted to give my best efforts forward," Sin said on Saturday. "I'm happy that I scored a goal in my last match with the club."

Football fans, however, can still see Sin on the pitch because he will be playing for the military club Sangju Sangmu after his five-week basic military training ends. Sangju are also playing in the K League Classic. 

"I will keep up my performance with Sangju, too," he said. "But I will not forget to applaud if Seoul win both the ACL and the K League Classic."

Seoul head coach Choi Yong-soo lamented that Sin will no longer be with the club at least for two years. Choi insisted on signing the midfielder during this past offseason even though he knew Sin had to depart during the season.

"Our players often had slow starts to seasons and I wanted to Sin to fix this problem," Choi said. "But Sin performed better than my expectation. He will be remembered by our players and fans."

Choi's task now is to fill the void that Sin left. Though he hasn't specifically revealed what the solution will be, Choi said he is confident of maintaining the team's pace without Sin.

Fans will be able to see Choi's answer on Wednesday at Seoul World Cup Stadium, where Seoul host Thai champions Buriram United for their fifth ACL match in Group F. Seoul crushed Buriram 6-0 in their first encounter in February.

"The reason why our team is getting good results is that everybody has confidence that he can contribute to the team's victory," he said. "There are a couple of players in my mind who can carry out Sin's role."

Pundits expect that one possible way to address Sin's absence is using midfielder Lee Seok-hyun. The 25-year-old, who joined Seoul from Incheon United last year, can provide offense from the midfield like Sin.

"Lee is waiting for his chances to play," Choi said. "He is a player who can serve the same role as Sin."

Choi hinted that he can even give defensive midfielder Ju Se-jong a more offensive role. Ju, who joined Seoul this season from Busan IPark, is known for his passing skills and hard work in the midfield.

"Ju has a different style than Sin, but I believe he can do his job well when he is asked to play more offense," Choi added.

The 42-year-old coach has been using a 3-5-2 formation as their main tactic, but without Sin, Seoul also have the option of using three forwards upfront in a 3-4-3 formation. Former Arsenal striker Park Chu-young has been a backup to Adriano and Damjanovic in Seoul this season, but with Park's proven ability to also play as an attacking midfielder, Choi could position Park below the two foreign strikers to support them.

"I prefer putting three midfielders, but in some situations, I could use Adriano, Damjanovic and Park all at the same time," he said. (Yonhap)