To be clear:
The fact that the band is by common agreement the owner of the recording rights to the original album master is totally irrelevant to who owns the musical composition rights of the individual songs on the recording. 
Unless there are some unspecified factors outside of what you have said so far, you own the musical composition rights to the melody line and lyrics that you wrote. You can do anything you want with that melody and lyric only. 
Unless there are unspecified factors not mentioned above, the presumption is that the actual author/inventor of the guitar part owns the musical composition rights to the guitar part. The fact that the guitarist has not registered his composition rights to the guitar part as a separate composition is totally irrelevant to his ownership of those rights. Unless you can show that some written agreement signed by the guitarist assigned (transferred/sold/conveyed) his composition rights in the guitar part to you, the presumption is that he still owns those rights. Under the presumption that the guitarist owns the musical composition rights to the guitar part he wrote, you use his composition at risk of infringing his musical composition rights in the guitar part if you copy it in any significant way when you do your own performance, unless you obtain his license to do so. If the guitarist as a person refuses to grant you a license, you can obtain a compulsory license, because he, like you, made his composition rights subject to compulsory licensing by permitting his work to be published as a recording. But you will have to pay the statutory fee for each copy made under a compulsory license. 
 
There are several ways that you could overcome the presumption that the guitarist owns the musical composition rights to his guitar part. And you may be able to mount a successful defense to a complaint of infringement, but bottom line, that is the presumption that you are going to have to refute in court. The way professionals in the entertainment business handle this kind of dangerous ambiguity is to get a license for what they want if it is cheaper than the risk of going to court, or by using original or different licensed material from another source instead of the iffy stuff. If it turns out that they could have used the material without a license, the cost of a not entirely necessary license is considered a wise investment nonetheless. An infringement lawsuit is going to expensive for everyone involved, and only one party is going to come out as a winner in the legal sense. Unless the monetary value of the new recording is going to be big, the outcome of a lawsuit will still leave everyone a financial loser. You can hope that your guitarist will be deterred by that  practical realization, or you can do what the professionals do and clear the rights you need. 

The determination of what constitutes significant copying or of what is sufficiently original to qualify for its own copyright in a particular instance is undefined in statute. The only way to find out for sure is to copy it and see if someone sues, then let the judge or jury decide. Precedents include tests to guide their decisions, but the tests are not as simple as "how many measures." A judge or jury will be asked to decide if there is substantial similarity in the works, and if so if enough of what was valuable in the original was appropriated that it is actionable. In general the line is much clearer for sampling of a recording than it is for copying music. That makes sense, in that it is nearly impossible to argue that the exact sounds captured by a microphone could be independently created, or accidentally duplicated. Bridgeport Music v. Dimension Films held that a sample of any length of a recording was infringement. The ordinary listener will be more likely to determine that a recognizable musical motif is appropriated, original, and valuable than that a drum pattern or base line is. Chord progressions are generally so common that they get a pass. Independent creation is an absolute defense against infringement, but substantial similarity and access to the original will generally be interpreted as proof of copying. So if I could prove that I was raised in a cider keg and only received gruel through the bunghole. with no other contact with the world prior to writing my version of Red October, even if it were word for word identical to Tom Clancy's work, I should be acquitted. You clearly do not have that option with band material. 

bapu

BobF
Don't forget that Fogerty got sued (and lost) in a case for plagiarizing his own work.

Urban myth as I see and remember it.
 
http://mentalfloss.com/article/27501/time-john-fogerty-was-sued-ripping-john-fogerty
 
In the original suit about the two songs, Fogerty won. Then he sought lawyers fees in a separate suit, which he lost. But, he went to The Supreme Court and then won the argument that he was due Lawyers fees.
 
He DID NOT lose the case involving the two songs which Zantz of Fantasy Records brought to trial.

I was lied to!!
 
Hey, it was still a huge hassle for him.