It's easier to just think of him as lazy or unmotivated of course. I remember people continually saying that he was "saving himself", which, while to a certain extent true of the post-2011 Messi, never really materialized, now did it? At first he was meant to be saving up for the title deciders and when that didn't quite pan out it became the WC... I wouldn't be surprised if some argued that he was saving up for the latter stages of the cup.
He's been physically declining for a little over a year now. Yes, his playing style changed structurally from 2011-12 onwards which caused him to cover far less ground and use his energy more sparingly, but from Messi's performances for Argentina in that time-frame (under Sabella as well, from his appointment in late 2011 until Messi's injury against PSG in early 2013) it was clear that he was still capable of playing energetically, on high-octane transitions. He gave some of his best performances (and certainly most consistent ever) in an Albiceleste shirt in that period -- easily rivalling and even bettering his form for Barça at that point. The series of injuries that followed throughout 2013 seem to have taken a toll, as much psychologically as physically perhaps, and there is a definite change noticeable in his (relative lack of) explosiveness.
My view on the evolution of Messi's WC performance:
1. In the first two games he was still finding his feet (also not helped by Sabella opting to start off in their 1st game against Bosnia with the less familiar 5-3-2 that they rarely used in qualifying), but made the decisive plays. Against Bosnia he showed that he still has some explosiveness with his goal. The game against Iran was less indicative because Arg had more than 70% possession and no space at all to operate in, with everything crammed just outside Iran's box. In the next two games he could show his explosiveness again (esp. against a very open Nigeria side), even if in a flash (the run that provided the winner against the Swiss in the 118th minute was the fastest sprint that Messi clocked in that game). Those two were also his best performances IMO.
2. At this point it became clear to Sabella that Gago was still not looking particulary match fit, so he brought in the far more defensive-minded Biglia to start against Belgium (Demichelis also came in for Fede Fernández who had been a total liability). This combined with the tournament-ending injury to Di María meant that Arg were now forced to prioritize DF. In the final three games (Belgium QF, Holland SF and Germany F) there was a very noticeable difference in approach to before. Arg dropped a lot deeper and played a lot more conservatively (now lacking even more ball carriers/playmakers from MF without Di María and Gago). Enzo Pérez did a decent job when he came in, but with Biglia there that was enough to ensure MF had nearly zero creativity if Messi didn't drop deep. This (forced) tactical shift did not play up to Messi's strengths, as he just doesn't have the stamina anymore to bridge that huge gap between DF and the forwards. Also the fact that he played 120 minutes twice in the run-up to the final didn't help.
The criticism that I've seen of him not putting in any effort would've been justified had it been in a situation such as the one a Barça fan on here labelled his "worst performance ever" (the CL QF v Atlético, an assessment I agree with by the way), where he clearly did not face this problem as he could stick close to the opp. box at all times. It's another story in games like the ones against Germany, where he was forced to cover 40 yards everytime the Argentine DF recovered the ball, just to get it into the forwards or get it near the opp. box. Had he been constantly pressing his opponents in the game he would've been carried off on a stretcher at HT -- as it stands he actually gave a quite energetic performance in the first 60 minutes, but then deflated completely (maybe also psychologically because of his missed chance at around the 54th minute I believe?) and in turn Arg just resorted to hoofing it long to reach their forwards.
In a nutshell, his physical endurance and explosiveness have significantly declined IMO. It's quite likely that this is the by-product of the amount of games he played between 2008 and 2013, which was then exacerbated by a string of muscle injuries. I even think that he'd already lost some of his original explosiveness because of his recurring muscle injuries in 2006 and 2007, but that's another discussion.