The great thing about WAR is it is essentially designed for this question. Whether or not one believes in it is another matter but at 73 wins the cubs were 15 wins short of the 88 wins it takes to make the wild card this year. The got the following fWAR out of the various positions
C - 0.9(Castillo 2.2, Whiteside -0.3, Rafael Lopez 0.0, Baker -1.0)
1B - 5.7(Rizzo 5.6, Valaika 0.1)
2B - 1.1(Bonifacio 1.9, Barney 0.4, Watkins -0.4, Baez -0.8)
SS - 2.9(Castro)
3B - 2.1(Valbuena 2.7, Olt -0.6)
LF - 0.4(Coghlan 2.2, Kalish -0.8, Lake -1)
CF - 0.5(Ruggiano 0.4, Alcantara 0.2, Sweeney 0.0, Szczur -0.1)
RF - -0.2(Soler 0.7, Schierholtz - 0.9)
SP - 12.9(Arrieta 4.9, Samardzija 2.0, Hammel 2.0, Hendricks 1.5, Wood 1, Wada 0.8, Jackson 0.5, Villanueva 0.3, Beeler 0.1, Doubront 0.0, Turner -0.1, Jokisch -0.1)
RP - 4.5(Rondon 1.5, Strop 1, Villanueva 0.8, Ramirez 0.8, Grimm, 0.6, Schlitter 0.2, Wright 0.2, Parker 0.1, Turner/Rusin/Straily/Baker/Jackson/Fujikawa/Jokisch/Vizcaino 0, Rosscup -0.1, Russell -0.2, Veras -0.3)
As an example, the Royals got 22.6 fWAR out of their hitters and 18.8 fWAR out of their pitching. The 2014 cubs were at 13.4 and 17.5 respectively. So, you're talking about a difference of 10.5 fWAR. I'd be hesitant to predict Rizzo and Arrieta matching their performance because neither has a huge track record. I think both can be good but this same level, I'm not so sure. That being said, the cubs had their entire OF and 2B at below average. C was sort of as well but that was mainly just Baker dragging Castillo down. An average Alcantara/Soler in CF/RF would be +3.7 wins. An average 2B is another +0.9. I'm not one to have a ton of faith in Baez but worst case you're still talking about Bryant at 3B and Valbuena moving over to 2B. So, assuming they replace baker with a replacement level catcher rather than a below replacement level you're talking about +5.6 wins of low hanging fruit on offense without going too crazy on projecting Soler, Baez, Alcantara, and Bryant to play well.
The bullpen should really be changed much as the only real "losses" are Villanueva and Russell which is kinda meh. The starters obviously lost 4 wins of Shark/Hammel. If we assume the cubs land Lester which is the speculation he was a 6.3 fWAR pitcher. Conservatively let's use his 2013 of 4.3 fWAR instead. He'd essentially offset the loss of those two. It's a bit cloudy from there. Arrieta might fall back some but given how good he was I don't think it will be that huge. However, 3-5 is a giant question mark. I'm a big Hendricks believer and I was that prior to the season. Conservatively, I think he can be a 2.5 win pitcher. Wood had a horrible year but I don't think he's that bad. I think he can be a 2 win pitcher pretty safely and if not him i think someone can step up and give you that be it Jackson figure out what the hell he used to be, Turner taking a step forward, Doubront....etc. If we conservatively call that 15 wins for SP you're talking a +2.1.
If we add up those you're at +7.7 of the 15 wins they were short of the wild card. Let's call a better manager in Maddon +2.3 wins giving you an even 10. But keep in mind that's just to get to the Royals level this year and they barely squeeked in.
