Moises Caicedo was a notable absentee from Chelsea's squad for the first leg of the Carabao Cup semi-final against Arsenal on Wednesday night. The Ecuador international is one of the Blues' most important players in central midfield but new boss Liam Rosenior was forced to deploy Andrey Santos alongside Enzo Fernandez against the Gunners because of his absence. Chelsea have struggled with suspensions so far this season, with five red cards forcing key players to miss out at various stages.As one of Chelsea's most important players, it was a bitter blow for the Blues to have to leave the former Brighton star out of their squad for Wednesday's game against Arsenal. The 24-year-old is already enjoying his best goal-scoring term at Stamford Bridge, having hit the back of the net on four occasions so far, while he also remains a physical and tireless presence in the middle of the park, often breaking up opposition attacks and recovering possession.With Caicedo absent from the squad, new manager Rosenior opted to pick Santos alongside Fernandez in midfield. The 21-year-old Brazilian impressed in the 5-1 thrashing of Charlton Athletic in the FA Cup on Saturday and has been rewarded with another start, while Joao Pedro is expected to play just behind Marc Guiu in attack. Estevao and Pedro Neto take the spots on the wings, while the likes of Cole Palmer, Reece James and Malo Gusto were all left out due to injuries. Liam Delap and Jamie Gittens are ill and also not included.Caicedo played a large role in helping Chelsea reach the Carabao Cup semi-finals, being named captain on the night by former manager Enzo Maresca for the 3-1 victory at Cardiff City in the quarter-finals. Two goals from Alejandro Garnacho and a strike from Neto ensured the Blues reached the final four, but a booking for Caicedo meant he would miss the first leg due to suspension.The midfielder had previously been booked in the Carabao Cup in the win against Wolverhampton Wanderers, which meant he picked up a one-game ban for receiving two yellow cards. Because he was carded for the second time during the quarter-final before his side had reached the semi-finals, he was handed a suspension.Speaking ahead of Wednesday's match, Rosenior lamented an illness bug that took out Delap and Gittens and a couple of small injuries that have ruled out James, Palmer and Gusto.He said: "We've been really unfortunate. Reece James took a knock on his hip and he's just not quite ready for this one. Unfortunately for Cole, he's obviously an outstanding player, he should be available for Saturday, but didn't quite make it for this one. "Liam was ill three hours ago. Jamie Gittens fell down with illness. Malo, hopefully for Brentford. Yeah, we've gone through the wars the last couple of days, but at the same time I think we're good enough to be competitive and try to win this game."Speaking about the threat Arsenal pose, he added: "They are very well organised in every aspect of the game. We have been tactically working on Arsenal since the minute I got into the building – including set-plays, which they are very good at. Arsenal are good at everything. I respect them and I think they will respect us."Add GOAL.com as a preferred source on Google to see more of our reportingWhile Chelsea will have to make do without Caicedo for the first leg of the semi-final with Arsenal, they will have their star midfielder available again at the weekend when they take on Brentford at home. With Chelsea down in eighth in the Premier League and in need of a positive result to push them back into Champions League qualification contention, a win is very much required. Brentford themselves are fifth ahead of Saturday's game after winning four of their last five league games.