The two sides had been playing out a thouroughly uninspiring encounter but the 22-year-old struck in time-added-on to send the Spaniards top of the group.
Betis now have eight points and are two clear of Lyon while Guimaraes are two further back in third.
The game got off to a tepid start and it was not until the 17th minute that Betis' Braian Rodriguez produced a chance for either side as he saw his shot sail over the bar.
Andre Santos was next to try his luck and his shot was much closer to producing the desired result for Guimaraes, with Stephan Andersen doing well to tip his long-range effort over the bar.
The contest had sprung to life and soon after Juan Carlos brought a save out of Renato Douglas in the home goal.
Nii Plange and Moussa Maazou both missed the target as the Portuguese side hunted for a lead but the deadlock could not be broken before the break.
Jose Nono saw a speculative effort skim wide early in the second half for the visitors but Andre and Maazou both saw attempts miss the target down the other end.
The second period had not been much better than the first but Betis went close to taking the lead in the 66th minute.
Jorge Molina saw his shot from 12 yards blocked behind by Douglas and from the resulting corner Jose Caro should have done better when he headed wide.
It looked like the game was going to fizzle out into a goalless draw but Joan Verdu released Chuli and the striker's shot across the the face of goal trickled over the line.